The undivided support of the state, which has a legitimate interest in helping make it possible for stable, settled families to produce new generations of citizens and thus to perpetuate itself. From the perspective of any system of rule, this is the purpose of marriage. Homosexual partnerships, which are incapable of producing children, thus are of no benefit to the state, and can only be an unproductive diversion of funds which otherwise would be allocated to the benefit of heterosexual marriages which can and often do produce children.
In the case where it's considered desirable to ensure that homosexual partners receive the same benefits of familial status (e.g. the commonly cited hospital visitation rights), without also including such partnerships in the financial benefits of heterosexual marriage, a federal civil union statute would easily suffice. Such things have been proposed many times, but invariably have been shot down from the left, rather than the right, as representative of "second-class citizen" status, just like "whites" and "coloreds" water fountains - a risible claim, especially in light of e.g. GLAD's explainer [1], in which, discounting the two already disposed of here, every reason cited in favor of "gay marriage", and against civil union, is at its base about money.
Such, at least, is the primary reason why I oppose the idea of "gay marriage", to say nothing of the Supreme Court's having recently discovered in the Constitution that there exists an inalienable right to same. This puts me in an apparently contradictory position, being that I am myself homosexual, but that contradiction exists only in the presence of an assumption that identity politics are the only valid sort.
Of course, to hear "gay rights" organizations tell it, that is precisely true, and we are not allowed to take a stance such as mine, or indeed any other that fails to correspond exactly with the current party line. To do so is derided [2] or pathologized [3] - or, of course, both. The possibility that we might have evaluated the cases for and against on their relative merit, and made our decisions accordingly, doesn't really enter into the discussion - and neither does the possibility that those who claim to speak and act on behalf of all homosexuals need not have their claims taken at face value by anyone, not even we whom they insist they defend.
[1] https://www.glad.org/uploads/docs/publications/cu-vs-marriag...
[2] http://www.out.com/news-opinion/2013/05/13/bret-easton-ellis...
[3] http://mashable.com/2014/06/25/internalized-homophobia-lgbt/...