I think marriage equality is inevitable.
The marriage contract is authorized, not sanctified, by the state. Contracts between responsible adults (leaving out most of the fetishism that is used for spurious argument) may or may not be "sacred" but should, nevertheless, be considered legitimate (separation of church and state). The "injury" that might be done to that "ideal" toward which the "orthodox" aspire by the commitments of some about whom they hold dear (same-sex marriage contracts) is relative to one's fear about how the "sin" of those commitments might despoil the religious "innocence" of society. There is no evidence, however, that such commitments negatively affect communities, except for the hysterical reaction from the intolerant.