I agree that society evolves, as does evolution.Oh, KiMare
Society evolves else we'd be forever stuck in the dark ages. There was a time in early marriage legality when the wife was not considered a person, she was part of 'chattal', belongings. If marriage is only a 'cross-cultural constraint of mating behaviour' then no couple would be allowed to marry knowing that one or both of them are infertile. Taken to the extreme reductions which you imply then you yourself would not be allowed to have married, as genetically you're not singularly male; and from a psychological point how do you justify you yourself having kids when you can not give them an entirely male role model?
Marriage is gestaltian ie. it is more than the sum of its' parts. Yes it has traditionally served the function of procreation but reality (that thing you think you're so fond of) is it is more than that. Nowadays plenty of couples have children before they marry, it's quite common to have the kids at the wedding as flower bearers.
Stopping gay couples from marrying is not going to stop homosexuality and neither is it going to stop them having (biological surrogates or adoption) having children; any more than it stops single people having children if they want.
Think about it, you're obviously legally male or else you wouldn't have been able to marry your wife. What if with a slight differentiation you had been registered female. Assuming you married out of love, then you would be left wanting a SSM. Changing marriage to define two people instead of man and woman would avoid this problem, not only for gays/lesbians but for transgenders/intersexed too.
The rest is BS.
Marriage defines a distinct and unique relationship, as does family. Why are you unable to establish and relish your own?