What non religious reasons are there for banning and not recognizing gay marriages?

anyone? anyone? the only one i can see is they cant have kids, which they could with a surrogate, and many heterosexual couples cannot have children either. i mean marriage is a governmental institution, not a religious one, what non religious reasons are there for denying two people equality because of their sexual orientation... it seems down right un-american to me.. and for all who cry tradition, some traditions need changing... i say this all as a straight man too