When the Supreme Court hears oral arguments on the constitutionality of the Defense of Marriage Act (DOMA) later this month, many casual observers will focus on what they call the fairness of redefining marriage. Interestingly, though, the dispute at the heart of the DOMA case could have been avoided had Congress enacted fairer tax reform years ago.

Consider the facts: In 2010, Edith Windsor sued the federal government, demanding a refund on taxes she paid upon inheriting the estate of her same-sex partner.

The couple had obtained a marriage license in Toronto three years earlier. New York state, where they resided at the time, did not grant marriage licenses to same-sex couples. The state did recognize their foreign marriage certificate, but the federal government did not.

And so when her partner died in 2009, Ms. Windsor was forced to pay over $350,000 in federal estate taxes because she didn’t qualify for a marital exemption.

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The lawsuit before the Supreme Court, set to be heard on March 27, seeks to strike down the section of DOMA that defines marriage for federal purposes as the union of a husband and a wife. In other words, to avoid this tax burden, the lawsuit seeks to redefine marriage.

Indeed, arguments for redefining marriage frequently hinge on questions of government benefits to same-sex couples. But more sensible solutions are possible. As this case shows, society can remove this burden without rushing to abolish marriage as the union of a man and a woman.

The Heritage Foundation has argued for eliminating the estate tax, popularly called the “death tax,” for more than fifteen years. In an influential 1996 report, Heritage argued that such reform was in line with the American dream and sense of justice. Tax law should not discourage savings and investment, nor punish hard work and thrift. Nor should it encourage Americans to consume now in order to avoid passing on wealth to loved ones because they would be taxed.

Killing the death tax would encourage economic growth, add jobs, and allow offices and factories to buy equipment that elevates productivity and wages. Lower capital costs mean new small businesses.

Best of all, such tax reform would treat all Americans fairly.

Ask yourself: What if Edith Windsor’s sister rather than a same-sex partner had died after living with her for decades? It would be arbitrary and thus unfair to redefine marriage to grant Ms. Windsor tax relief simply because she was in a same-sex sexual relationship while not granting that relief if she were in a similar, but non-sexual, relationship.

By contrast, nothing is unfair about government recognizing marriage as a union of a man and a woman (as a new Heritage report makes clear). Every marriage policy draws lines, leaving out some types of relationships; fairness forbids arbitrary line-drawing. Determining which lines are arbitrary requires answering two questions: What is marriage, and why does it matter?

Reflecting on these questions reveals good reasons behind DOMA and the decisions of forty-one states to protect marriage as we always have known it. Marriage exists to bring a man and a woman together as husband and wife to be father and mother to any children their union produces.

Not every married man and woman will produce new children, but every new child comes from the unique union of a man and a woman. Every child needs a mom and a dad. Marriage helps make men and women responsible to each other and to any children they might have. And as ample social science has shown, children tend to do best when reared by their mother and father.

Government recognizes marriage because it is a natural institution that serves the public good. A study by the Brookings Institution finds that $229 billion in welfare spending between 1970 and 1996 can be attributed to the breakdown of the marriage culture and resulting social ills: teen pregnancy, poverty, crime, drug abuse, and health problems. A 2008 study found that divorce and unwed childbearing cost taxpayers $112 billion a year.

Redefining marriage does not really “expand” the existing understanding of marriage. It rejects the anthropological truth that marriage is based on the complementarity of man and woman, the biological fact that reproduction depends on a man and a woman, and the social reality that children need a mother and a father. Redefining marriage to abandon the norm of male–female sexual complementarity would also make other essential characteristics—such as monogamy, exclusivity, and permanence—optional. But marriage cannot do the work that society needs it to do if these norms are further weakened.

We should seek to craft good public policy that benefits all Americans. That doesn’t require redefining an institution so central to civilization as marriage. Sometimes it just means repealing a tax.