I'm sorry, but I fail to see how a woman who's had a hysterectomy (for instance) could get married, knowing perfectly well that she can't have kids (unless stem cell engineering magically improves, and she can regrow a uterus and all that jazz--but then perhaps a gay man could grow extra bits as well).

And why do straight people need state-instituted marriage anyway? Clearly, it is far from necessary for having kids. There's plenty of cheating spouses as well, so only the woman can know if the kid is hers anyway. DNA testing seems a lot more reliable for determining family ties (except in the case of identical twins sharing sex partners or having sex with other identical twin pairs). DNA testing is already used extensively for child support and such.