Orin, I also want to respond to your first very perceptive question: " If marriage is so innate, why should we expect a small number of same-sex marriages to have a significant effect on how the majority in opposite sex relationships behave?"
I don't think marriage is a universal human institution because marriage is innate. The forms of marriage differ so wildly, cultural variations are huge. You only have to go into the inner cities to see that marriage is not innate. It can disappear.
Yes, there are things in human nature that help sustain it (e.g. a pair bonding preference; sexual jealousy) but others that undermine it (e.g. men's subjection for much of their life to powerful, indiscriminate and rather impersonal lusts).
But fundamentally marriage is sustained by culture, not biology. Why then is it universal? Because it is the answer to an urgent problem that is biological and innate: sex makes babies. Nature alone won't connect fathers to children. Children need a society in which both men and women are committed to their care.
When anthropologists in the thirties went out into the vanishing world of human diversity, the reason they found marriage everywhere is that societies that do not hang onto the marriage idea do not survive very long.
But marriage in a particular society is not inevitable; death by sexual disorganization is always an option. Happens quite a bit actually. cf. Roman empire.
So in one sense I'm not worried about marriage. In spite of the progressive mythology that the drive to gay marriage is the irresistible wave of the future, I'm quite confident that 200 years from now, we're not going to be living in a world where gay marriage is the norm.
I'm just not sure of the place of Western civilization in that future world.
This is part of what I mean when I say if I'm right (big if!), gay marriage is not in the interests of gay and lesbian people. We may need to find some new solutions to the legitimate social needs of gays and lesbians. But don't mess with marriage.