This story about incest exposes the fact that consent alone is not enough to ground a sexual ethic. Nor is it sufficient to define who should be allowed to marry. It also shows that when proponents for “marriage equality” say that any two people who love each other ought to have the right to marry, they don’t really mean it. The possibility of state-sanctioned incest proves that.

I just finished reading an article in New York Magazine that I wish I could unread. It is an interview with a young woman engaged to marry her own father. I am not even going to link the interview here–it is just too vile and disturbing to share. You can read USA Today’s coverage of the situation here if you are interested.

I mention the article because it raises some issues for the “marriage equality” movement. The sexual revolutionaries have been pushing to normalize any an all sexual relationships so long as they are between consenting adults. In effect, the only constraint on sexual morality is consent. Flowing from that, is the idea that two consenting adults who love one another should be able to “marry” one another even if they are of the same sex. To deny them their right to marry is denying them equal rights. Or so the argument goes.

But this story of an incestuous relationship exposes the weakness of this reasoning. If consent really is the only sexual norm, then we are left with no moral or legal grounds to deny recognition to incestuous unions. We have no response to those demanding “marriage equality” for those who have so-called “Genetic Sexual Attraction.” If consent is the only constraint on sexual relationships, then there are all manner of taboo relationships that now must be accepted into the mainstream. Incest would only be one variation.

Proponents of “marriage equality” know that this is an implication that they cannot live with. And it is why, for instance, that Samantha Allen argues at The Daily Beastthat there is no such thing as consensual incest. She argues that even if both parties are of age, the parent-child relationship always has an unequal power dynamic that renders consent impossible.

But here’s the problem with Allen’s reasoning. Even if we grant her case on this point (that consent is impossible between parent and child), one can imagine that consent would be possible in other incestuous scenarios. What if a brother and a sister–both over the age of consent–decided to form a union? Or a brother and a brother? Or a sister and a sister? Wouldn’t we still sense something amiss with those unions as well? I think that we would, but our moral repugnance for such relationships would have very little to do with consent. In fact we would be right to oppose such relationships even if both parties consented to it.

This story about incest exposes the fact that consent alone is not enough to ground a sexual ethic. Nor is it sufficient to define who should be allowed to marry. It also shows that when proponents for “marriage equality” say that any two people who love each other ought to have the right to marry, they don’t really mean it. The possibility of state-sanctioned incest proves that.

Denny Burk is Associate Professor of New Testament and Dean of Boyce College, the undergraduate arm of the Southern Baptist Theological Seminary. This article first appeared on his blog and is used with permission.