Incestuous connections lead to an overlap of family relationships and social roles and thus to a disturbance of a family bereft of [clear] assignments. … Children of an incestuous relationship have great difficulty finding their place in the family structure and building relationships of trust with their next caregivers. The vital function of the family for the human community … is crucially disturbed if its ordered structure is shaken by incestuous relations.
Liberals tend to recoil from such arguments. They fear that a movement to preserve the "family unit" would roll back equal rights for homosexuals. But that doesn't follow. Morally, the family-structure argument captures our central intuition about incest: It confuses relationships. Constitutionally, this argument provides a rational basis for laws against incest. But it doesn't provide a rational basis for laws against homosexuality. In fact, it supports the case for same-sex marriage.
When a young man falls in love with another man, no family is destroyed. Homosexuality is largely immutable, as the chronic failure of "ex-gay" ministries attests. So if you forbid sex between these two men, neither of them is likely to form a happy, faithful heterosexual family. The best way to help them form a stable family is to encourage them to marry each other.
Incest spectacularly flunks this test. By definition, it occurs within an already existing family. So it offers no benefit in terms of family formation. On the contrary, it injects a notoriously incendiary dynamic—sexual tension—into the mix. Think of all the opposite-sex friendships you and your friends have cumulatively destroyed by "crossing the line." Now imagine doing that to your family. That's what incest does. Don't take my word for it. Read The Kiss. Or the sad threads on pro-incest message boards. Or what Woody Allen's son says about his dad: "He's my father married to my sister. That makes me his son and his brother-in-law. That is such a moral transgression. I cannot see him. I cannot have a relationship with my father …"
Homosexuality is an orientation. Incest isn't. If the law bans gay sex, a lesbian can't have a sex life. But if you're hot for your sister, and the law says you can't sleep with her, you have billions of other options. Get out of your house, for God's sake. You'll find somebody to love without incinerating your family. And don't tell me you're just adding a second kind of love to your relationship. That's like adding a second kind of life to your body. When a second kind of life grows in your body, we call it cancer. That's what incest is: cancer of the family.
