On the topic of the impact of growing up in a same-sex parent household on kids, my father is on wife #3 and my mother is looking for a husband of the same iteration. I'd say the odds of the dynamics of a married gay couple doing any more "damage" to a kid than was done to me or many kids similarly situated is unlikely. If people are so concerned about the impact on children of growing up in a same-sex parent household, championing or at least maintaining an accepting, live and let live attitude, such that these kids wouldn't have to grow up in a society that made them feel like outcasts or oddities, would go a long way toward reducing any such impact.
My brother-in-law is gay. He's a great guy. He's great with my girls - loves 'em to death. I wouldn't see him denied the right to have kids of his own just because he was born attracted to a different type of consenting adult, and I wouldn't see him denied the right to marry for that reason, either, especially if he can offer a loving, stable home, which is apparently becoming increasingly rare for children these days.
As for the sanctity of the institution of marriage, I think heterosexuals have already done too much damage to it for homosexuals to have much impact at this point.
For goodness sake, let 'em drink from the same water fountain, already.