Apropos of the 9th Circuit’s ruling on Proposition 8, rather than raise the same arguments again we can just turn to Ampersand’s analysis of why same-sex marriage won’t make incestuous marriage more likely, particularly his brilliant analysis of why there is a rational basis for prohibiting incestuous marriages between adults:
In my case, I’d first say that marriage is a kin-making institution, which by definition transforms two unrelated (or at least not closely related) people into close kin. It makes no more sense for marriage to turn close kin into close kin than it does for an alchemist to transform gold into gold. It’s already gold.
Second, I’d point out that limiting incestuous marriage to adults would not solve the problem of child abuse, and could conceivably make it worse. Legal recognition of incestuous marriage could give sexually abusive fathers (or mothers — but in most cases, sexually abusive parents are fathers) a strong incentive to sexually abuse their daughters, in hopes of crushing her will before she comes of age, so she’ll agree to marriage.
Finally, I’d argue that child health and child abuse are not the only harms of incest. Importantly, legalizing incestuous marriage between the infertile would transform currently existing families, by introducing the possibility of marriage into relationships that have never had that possibility. It fundamentally changes the relationship between father and son, or between sister and brother, if we add the possibility of marital union to those relationships.