In order to accommodate same-sex couples within the institution of civil marriage, marriage must be re-defined to mean simply a union of two people. The way these unions choose to express intimacy suddenly must become a thing of public indifference just like one's choosing of religion. But this is just insane. Coitus is not like any other act of intimacy, be it frottage, cuddling, or an intense, soul-baring conversation about Proust. Coitus can and often does result in children of whom the state prefers not to be primary caretakers.
It is for this reason that only heterosexual relations are subject to Paternity Laws. If the state has a compelling interest in discouraging irresponsible heterosexuality by means of Paternity Laws, then it clearly has an interest in encouraging responsible heterosexuality. But the re-definition of marriage would make that which sets heterosexual relations apart into simply one among many random acts of intimacy, and that's just nonsense.
If equality demands that we treat heterosexual and same-sex relationships as fundamentally the same, then we must abrogate Paternity Laws forthwith: It violates the sacred dictates of equality that a gay man can enjoy a one-night-stand without fear of a Paternity Suit but a straight man cannot. Same-sex "marriage" requires the relativization of coitus, and that is a singularly stupid, insane, and absurd idea.