In those states and countries where same-gender couples can marry, their marriages are EXACTLY the same as everyone else's.What hasn't changed?
Ss couples are still only ever a mutually sterile, pointlessly duplicate gendered half of marriage.
Clearly not the same.
So your definitions--"marriage is a cross cultural constraint on evolutionary behavior" and "same-sex couples are still only a mutually sterile pointlessly duplicate gendered half of marriage"--they simply don't apply.
I'm sure that galls you to no end. Either get on board or step aside.