It's only a matter of time before a church bless a marriage between a father and his daughter.
.... or to bless a marriage between a married man and an additional wife. :embarrass
I think once Homosexual Marriage becomes mainstream poligamy may be next, not incestual marriage.
Personally I oppose state recognized marriage and special rights for married people altogether (it is discrimination against single people) so I really have no horse in this race.
My outsider's point of view solution would be this:
Have the state recognize something we call "civil union" that carries with it all the legal rights marriage now does. Have this "civil union" be a contract that any pair of people can enter (or maybe threesomes or foursomes). Men can enter this with women, with men, with friends, with siblings, whatever. Have no romantic or spiritual attachment to it whatsoever.
Then have "marriage" be a spiritual bond between people, with no legal implicaitons or rights attached to it. Then various religions may recognize (or reject) people as being in marriages however they like, and they need not agree with one another. The pious church or mosque can then reject homosexual marriage while the liberal one accepts it. The mormons or muslims could have poligamous marriage and the christian church need not recognize it.
I see this homosexual marriage issue as opening up a broader issue of the separation of church and state. The church has no business telling the state who should be entitled to certain legal rights. And equally, the state has no business telling the church who must be recognized as being in a spiritual union.
Would you accept this compromise? If not, why not?