Will same-sex marriage cause harm to opposite-sex marriage? It’s one of the most enduring questions surrounding state and national legal decisions about marriage.

But the question itself is empirically unanswerable any time soon. We are arguably years away from gathering quality longitudinal, nationally representative data on the matter. And even then, assessing—let alone agreeing upon—causation will remain difficult. Same-sex marriage may, after all, be a later-stage symptom of the general deinstitutionalization of marriage rather than, as many assert, a cause of it. So the question remains less an empirical one than a theoretical one at present.

And yet we can build plausible hypotheses about the broader influence of same-sex marriage by looking around the neighborhood—that is, at what we already know about gay and straight relationships, about what’s happening to marriage, the mating market, and how institutions change.

In the simplest sense, of course, same-sex marriage won’t alter the institution for everyone, because nothing ever happens to “everyone” in reality, or in social science data. Associations, probabilities, and educated guesses are the best we can establish.

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Lots of changes in marriage have, and will continue to, come about. What should we expect next? That’s the question Liza Mundy pursues in her cover story in this month’s Atlantic Monthly. “The Gay Guide to Wedded Bliss” explores the ways in which same-sex marriages may very well school those of us who have already entered—or someday will enter—the hallowed and embattled institution. Mundy is confident that such unions “could help haul matrimony more fully into the 21st century,” and that real influence is possible. This is in stark contrast to the politically tailored message that same-sex marriage will change nothing.

“What if same-sex marriage does change marriage, but primarily for the better,” she wonders aloud. How would this work? By giving us “another image of what marriage can be,” she asserts. What sort of image? According to Mundy, it’s the cardinal virtue of equality, or egalitarianism. Sameness and fairness.

Before we prematurely declare this image worth mirroring, consider for a few moments the side effects Mundy identifies on the way to the egalitarian utopia she praises. Three in particular stand out.

Women’s Unions Remain Unstable

Mundy first explores the instability—or “dynamism,” if you’re an optimist—of lesbian relationships. Don’t want a divorce, Mundy asks? “Don’t marry a woman,” she warns. University of British Columbia economist Marina Adshade concurs. The author of a new book—Dollars and Sex—on the fascinating economics of relationships, Adshade notes the dismal science around breaking up in Britain, where “62% of civil union dissolutions (i.e., divorces) in the UK are between women despite the fact that lesbian relationships only represent 44 percent of civil partnerships in that country.” The greater instability, she reasons, is simply about gender differences in relationship preferences, and nothing more. I tend to agree.

The elevated breakup rate among lesbian couples has been an open secret for a long while. Even NYU sociologist Judith Stacey—no fan of marriage in general—noted it back in 2000 in small, nonrandom studies of upper-middle-class, educated white lesbian parents, demographic factors historically associated with stability rather than dissolution. Stacey and her colleague Tim Biblarz attributed the instability to, among other things, the participants’ “high standards of equality.” In Mundy’s words, “women are just picky, and when you have two women, you have double the pickiness.”

Writing in Slate last year, June Thomas highlights this predilection toward shorter, intense relationships, and wonders whether the marital shoe actually fits:

I’ve noticed that my visceral anti-marriage animus is particularly strong when I hear twentysomething lesbians talking about their wives and fiancees. Are they really going to mate for life, like swans in sensible shoes? That seems attractive at 35, but at 25 it’s positively Amish. Worst of all, it threatens the continued evolution of a talent perfected over the millennia as our relationships have gone unrecognized by church and state: a gift for breaking up. Lesbians tend to bond intensely and often.

The pattern is evident in the Netherlands as well as Norway and Sweden, where Mundy notes that the risk of breakups for female partnerships more than doubles that found in male unions. The actual study she cites estimates that in Sweden 30 percent of female marriages are likely to end in divorce within six years of formation, compared with 20 percent for male marriages and 13 percent for heterosexual ones. The study’s authors suggested that lesbian couples may be more “sociodemographically homogamous” than other couples, a fancy term for “too similar,” and speculate that this may be conducive to a high level of dynamism, but perhaps not to the kind of inertia that has long been a hallmark of marital stability.

Using nationally representative data on American relationships, Stanford demographer Michael Rosenfeld reported at the 2012 annual meeting of the American Sociological Association that lesbian couples report higher relationship satisfaction alongside higher break-up rates. The greater comparative instability among lesbian couples persists even after a lengthy series of control variables is included, including the presence of children.

Female and Male Homosexuality: Apples and Oranges

A second distinction—a deficit, most would agree—is the too-settled nature of lesbian couples’ sex lives, once ignominiously characterized as “lesbian bed death” by sociologist Pepper Schwartz. (Mundy notes how profoundly Schwartz—an ardent fan of same-sex marriage—regrets being identified with the phrase.) There is much less documentation of this phenomenon, and the Schwartz reference is now decades old (and was derived from a nonrandom study to begin with). So what exactly is known here?

Frankly, not a lot, due perhaps to the first problem Mundy notes—that it’s challenging in population-based datasets to track the long-term sexual frequency of a small group (lesbian Americans in relationships) not known for the longevity of their unions. Schwartz, Mundy writes, “posits that lesbians may have had so much intimacy already that they didn’t need sex to get it.” Heterosexual women, on the other hand, are more apt to find intimacy with a partner via sex, even if they’d prefer their intimacy in other ways. Sex is a common “love language” for men, pop psychologists are quick to assert. It should be noted, too, that egalitarian couples tend to report less frequent sex—and women report lower sexual satisfaction—than couples who exhibit “more traditional household arrangements.” Perhaps sameness and fairness, however represented, have their unintended consequences.

Data from the fourth wave of the National Longitudinal Study of Adolescent Health, now interviewing young adults in their later twenties and early thirties, reveal this basic comparative truth—that the frequency of sex in same-sex relationships is predictably higher among men than women. “Bed death,” however, may be a bridge too far. Here again, not enough data just yet.

While Mundy notes the decline of sex in lesbian relationships, she also describes the diversity of sexual partners that is more characteristic of gay men’s lives than of others. Chatter about monogamy and “monogamish” has no doubt increased in frequency, despite the political risk to same-sex marriage efforts. It took a bold leap forward with Mark Oppenheimer’s biography of sex columnist Dan Savage, which appeared in the New York Times Magazine in June 2011. In it Savage, who is gay and legally married with an adopted son, admits he and his partner have had nine extramarital partners between them, and concludes that monogamy just doesn’t fit very many men.

The reality of monogamish gay unions is emerging in scholarly circles as well. Once a closeted topic, it feels safer for many social scientists to note this phenomenon, now that same-sex marriage is becoming a reality in more and more locales. One nonrandom study of 566 gay couples revealed that only 45 percent of them had verbally agreed to monogamy, fewer than the share who agreed to be “open.” So what do the population-based data say? That it’s true.

According to Add Health data on never-married heterosexual, homosexual, and bisexual men and women ages twenty-four and up, gay men are far more likely than straight men to say “yes” to two questions—during the time you and your partner have had a sexual relationship, has your partner (as far as you know)—or you yourself—ever had any other sexual partners? Just under 30 percent of gay men suspect their primary partner of being “monogamish,” compared to 15 and 20 percent of heterosexual men and women, respectively. Whether such external sexual partnerships were permitted or not is unknown. More, of course, have strayed themselves—43 percent of homosexual men and 26 percent of heterosexual men. It remains true whether the relationship is serious—cohabitation—or less serious, such as dating.

Although the frequency of sex in same-sex relationships is higher among men than women—as noted above—it is not higher than reports from opposite-sex relationships. Thus a quest for novelty, not simply greater frequency, seems to distinguish men’s relationships from those involving women.

So gay men have more partners, but no more sex overall, than straight men. Why? In keeping with sexual economics expectations, it’s not that gay men necessarily wish to have more sexual partners than straight men. It’s that they are far more apt to be in relationships that permit them because their relationships are with men, who tend—on average—to be more sexually permissive than women. Men make poor gatekeepers when it comes to attractive others’ sexual advances. That is not news. Thus the tension around nonmonogamy is simply not as dynamic among many gay unions as it is among those with a man and a woman. NYU sociologist Judith Stacey, interviewed in the New York Times Magazine article on Savage, agrees:

“They are men,” she said, and she believes it is easier for them—right down to the physiology of orgasm—to separate physical and emotional intimacy. Lesbians and straight women tend to be far less comfortable with nonmonogamy than gay men.

I didn’t say it, but I believe it, and the data support it.

Stacey notes this gatekeeping function as well in her ethnography of gay relationships in Los Angeles, quoting early gay activist and journalist Randy Shilts’s remark that among gay men there was “no moderating role like that a woman plays in the heterosexual milieu.” Furthermore, Shilts, author of And the Band Played On, asserted that “so much of the gay community’s sexuality . . . seemed more defined by gender than sexual orientation.”

The Add Health data concur, revealing that the subject of monogamy and its discontents is not limited to gay men. Shilts described how “some heterosexual males privately confided that they were enthralled with the idea of . . . immediate, available, even anonymous sex…if they could only find women who would agree.”

This phenomenon, I suspect, has the unique potential to genuinely shape or affect heterosexual marriage. Why this one? Because it’s the only one of the side effects Mundy notes that is exclusive to men. And why does that matter? Because—as I’ve written elsewhere—men have gained a decided advantage in the wider mating market.

Men’s Interests Dominate the Mating Market

The terms of contemporary sexual relationships favor men and what they want in relationships, not just despite the fact that what they have to offer has diminished, but in part because of it. The supply of marriageable men (but not women) has shrunk—for lots of reasons left unexplored here—leaving the balance of those who are more marriageable (e.g., more educated, wealthier, more stable) with more power than ever before to realize their wishes in their relationships. Meanwhile, women no longer need marry to thrive, but most still wish to marry. As a result, women find themselves in a weaker position in the marriage market, competing with other women for desirable men. On the other hand, they have more power than ever—and often employ it—to leave relationships that have gone sour. Big difference. If women’s position in the wider mating market—and inside relationships—was more advantageous, they could not only generate fine and stable relationships but also eschew sex—if they felt like it—without relational consequences for intimacy or male unhappiness. But that, we know, is not the case.

A key here lies, strangely enough, in the legitimacy that straight women already accord gay men’s unions. (Women support same-sex marriage at levels well eclipsing that of men.) Why does this matter? If gay marriage is perceived as legitimate by heterosexual women, it will eventually embolden boyfriends everywhere (and not a few husbands) to press for what men have always wanted but few were allowed: sexual novelty, in the form of permission to stray without jeopardizing their primary relationship.

Why is straight women’s acceptance of gay marriage important? Because adultery doesn’t work the same way in a significant share of their unions; instead of a single standard, couples negotiate (and often renegotiate) what their standard will be. It’s why Dan Savage can call nine extramarital partners being monogamish rather than serial cheating. Social theorist John Milbank asserts that when the definition of adultery must be tweaked, the exclusive sexual union risks ceasing to be perceived as having unique relevance—that is, not crucial—for marriage in general. We’re not there just yet, but the bridge is definitely under construction.

While seated next to an Army attorney recently on a flight to Washington, DC, I asked how his office would prosecute same-sex adultery cases in a military long known for defining the crime by penile-vaginal penetration. His response? “We’re awaiting orders on that.” My hunch? The decriminalization of adultery, as has long since occurred in civilian life.

A similar process has played out before: There’s been a wide and comparatively recent uptake of anal sex in heterosexual relationships, boosted by the normalization of gay men’s sexual behavior in the American (male) imagination. If men’s mating market position was weak, however, anal sex would remain wishful thinking. But the survey numbers suggest otherwise: All forms of sex are on the rise in relationships in general, despite women’s lower average interest in diverse sexual practices. Next up: sexual non-exclusivity.

The foothold gay marriage has already accomplished is thus a very significant one. The Army attorney’s simple response is a signal of institutional change afoot. Rights, expectations, and duties in marriage are up for grabs, as Mundy’s article—and the state of the social science here—attest. For all who enter it, very little about marriage is “second nature” anymore.

Mundy of course is hardly the first to speculate about the matter. Twenty-one years ago already, prominent British social theorist Anthony Giddens conjectured in his book The Transformation of Intimacy that marriage will become increasingly beholden to, and modeled after, what he calls the pure relationship—togetherness unfettered by material or psychological dependencies and mutual obligations. Gay and lesbian couples would exhibit this type of relationship best, he predicted. Marriage, then, is still a contract of sorts, but now much more open to negotiation of its terms, including sexual access.

So what can we conclude?

Many libertarians and conservatives, including Britain’s Prime Minister David Cameron, assert that marriage is a conservative institution—which is true—one that will therefore function as such for those who enter it, whether gay or straight. While certainly the case for some, that claim is an unlikely future for many, not because gay or lesbian couples are liberal but because those in the driver’s seat of the contemporary mating market—men—are permissive. This, I predict, will be same-sex marriage’s signature effect on the institution—the institutionalization of monogamish as an acceptable marital trait. No, gay men can’t cause straight men to cheat. Instead, the legitimacy newly accorded their marital unions spells opportunity for men everywhere to bend the boundaries. Dan Savage will be proud.

Mark Regnerus is an associate professor of sociology at the University of Texas at Austin.