Gay marriage OTOH is the imposition of an entirely unprecedented definition of marital relations, the implication being that America is abandoning one of the cornerstones of western civilization: the nuclear family.
Actually, the cornerstone of western civilization, indeed of civilization generally, has been the extended family, not the nuclear family, and while we indeed have been abandoning that institution for the nuclear family for some decades now, you have utterly failed to explain how gay marriage imperils family, be it extended or nuclear.
To the contrary, the extended family is if anything in a slightly stronger position then it has been historically, at least in countries like America which originated in colonialism and subsequent interminable westward spread. That sort of mobility was only possible because of the nuclear family's strength, since it by definition meant the abandonment of extended family ties.
It imperils the nuclear family because it signals the government withdrawing it's favoritism for reproduction within the nuclear family.