Twisting the Family Tree

If Same-Sex Marriage is Legal, must Incest be, too?

Twisted Tree. Artist: Axel KristinssonOpponents of same-sex marriage sometimes raise the specter of legalized incest. For example, in the federal court ruling that upheld Louisiana’s same-sex marriage ban, Judge Martin Feldman writes:

[M]ust the states permit or recognize a marriage between an aunt and niece? Aunt and nephew? Brother/brother? Father and child?

In a brief to the U.S. Supreme Court, Utah state legislators argue that the legal reasoning that declared Utah’s ban unconstitutional would indeed allow incestuous marriages.

Read on for an explanation and critique of how Feldman and Utah lawmakers each link same-sex marriage to legalized incest.

Love and Marriage

Supporters of same-sex marriage say that marriage isn’t defined by the sex of the spouses. The spouses’ loving commitment is what matters. For instance, the federal appeals court that reviewed Utah’s same-sex marriage ban suggested that the ban denied “state recognition of [the] love and commitment of same-sex couples.”

Judge Feldman, defender of Louisiana’s ban, argues that marriage can’t be defined as just a loving, committed relationship. Partners in an incestuous union could be “committed to love and caring for one another,” says Feldman. Yet same-sex marriage proponents don’t endorse legalizing marriage between close relatives.

If a mutual loving commitment is a couple’s core qualification for marriage, then how do marriage equality advocates hold the line against incest? Genetic risks won’t disqualify incestuous couples who are infertile, same-sex, or related only by adoption. So, must equality advocates deny that close relatives can make the same loving commitment that unrelated couples can?

No, same-sex marriage supporters don’t need to argue that incest is loveless or faithless. What’s crucial is to demonstrate that, gender and sexual orientation aside, same-sex and opposite-sex couples are the same. This fundamental equivalence is, I believe, what the appeal to love and commitment tries to establish. If same-sex and opposite sex-couples are basically alike, then same-sex marriage doesn’t lead to legalized incest anymore than opposite-sex marriage does.

Traditional marriage advocates claim that gender and sexual orientation can’t be ignored. Here are the standard objections from traditionalists.

  • Same-sex couples can’t procreate.
  • If they become parents (by adoption, assisted reproduction, etc.), same-sex couples provide inferior—even outright harmful—care to their children.

Here are the standard replies from marriage equality advocates.

Is marriage necessarily linked to procreation? Are gays and lesbians unfit parents? These questions advance the same-sex marriage debate more than inquiries about incest. Show the answers are “no” and “no” and the debate is largely done.

Judge Feldman links same-sex and incestuous marriages by defining  marriage as any mutual loving commitment. However, marriage equality advocates don’t have to accept that definition. The movement’s rhetoric may sometimes suggest that “marriage = love”. But the movement’s stance needn’t be simply that marriage is love and love is the same whether it’s same-sex or opposite-sex. Emphasize why the love is said to be the same: a couple’s gender makes no principled difference whatsoever, neither in love nor in any other vital respect. If “same-sex” versus “opposite-sex” changes nothing essential, we have (nearly) literal marriage equality: “same-sex marriage = opposite-sex marriage.”

Traditionalists need to show why saying marriage is gender-neutral leads to letting close relatives marry. After all, the man-woman rule doesn’t forbid incestuous (e.g., brother-sister) marriages. Why would dropping that rule mean we have to permit those unions?

Morals and Laws

Utah lawmakers propose an answer. Restrictions on gender and genetic/familial relatedness both rely on “moral standards,” say the lawmakers. In their view, marriage is a moral institution. Bans on same-sex and incestuous marriages are based “primarily upon a legislative judgment about what is acceptable in civilized society.” Reject that judgement as a basis for laws against same-sex marriage and you leave no basis for laws against incestuous marriage.

A federal appeals court rejected Utah’s same-sex marriage ban. The three-judge panel ruled that marriage is a fundamental right, and fundamental rights trump the moral disapproval of the majority. The ruling’s analysis mirrors Lawrence v. Texas, the 2003 Supreme Court decision that quashed sodomy laws, a form of morals legislation, for violating privacy rights.

In response, Utah lawmakers echo Justice Antonin Scalia’s dissent. Scalia called Lawrence “the end of all morals legislation,” including laws against adult incest. The lawmakers also appear wary of a right to marry a same-sex partner. They call this right “newly discovered” and deny that it should supersede “Utah’s moral judgments regarding the best interests of its children.”

That last quote reveals the linchpin of the Utah lawmakers’ argument: the effects of same-sex marriage on children. Will marriage equality harm kids? This question invites scientific judgments, not just moral ones. In fact, to defend Utah’s domestic relations laws, the lawmakers don’t just appeal to what Justice Scalia calls “moral feelings.” They cite empirical data, such as “rates of upward [economic] mobility” among Utah’s children.

An empirically informed debate about same-sex marriage is what we can and should have. And if research on the children of lesbian and gay parents shows that same-sex marriage is no threat to child welfare, moral standards won’t be what’s rejected. Ignorance will be.


Neither Judge Feldman nor Utah lawmakers succeed in linking same-sex marriage and legalized incest. Feldman relies on a potentially overinclusive definition of marriage that same-sex marriage supporters can likely reject. Utah lawmakers assume that allowing same-sex marriage means abandoning all standards of morality in marriage laws. But applying standards of empirical evidence looks to be what’s called for. Whether the evidence can change people’s views in such a morally charged debate is another matter.