First and foremost, "marriage" is a religious rite, developed under religious guidelines over the course of millennia. In my humble opinion, there is a distinct separation between the "sacrament" of marriage and the legal "contract" of marriage. For example, many religions will not recognize a marriage unless the ceremony is performed by the church and sanctified by a priest, minister or rabbi. It is, after all, the ultimate expression and example on earth of two becoming one. If done right, it is sacred, light years beyond contractual banalities.
If a gay couple goes as far as wanting to "marry", then I assume they view marriage in the spiritual sense. If this is so, then the odds of insanity on the part of the couple is rather unlikely. This being the case, I'm thinking incest of any kind would fall in to the anathema category.
Homosexuality and incest are not synonymous. Actually, when one views the statistics, one realizes that when it comes to incest, the Moral Minority should spend a little more time policing their own as opposed to worrying about things which don't involve them.