Thomas Bushnell has a blog entry about the unhappy fate of moderate positions on a certain class of social issues. To some degree I agree with what he says, but I think the subject needs deeper consideration.
Beneath his argument is a certain moral analogy: that issues of race, gender, and sexuality are, with regards to equity, essentially similar. I'd call this a conservative viewpoint in the sense that the primary contending positions have given answers in each of these three issues that assume this similarity. Bad guys said they were similar in that each issue was determined by essential differences in race/gender/sexuality; good guys said they were the same in being determined by essential equality.
For race, the equality position won. But this position has continued to be dogged by actual inequality in outcome, leading to more social problems to be fixed. This in turn has led to more radical solutions. So now the conservative position is that the law does enough now and there is nothing more to be done, the radical liberal positions vary but include such notions as reparations, and there is a quite assorted middle which thinks that the current structure of rights and laws is mostly OK but which considers a wide variety of activity or leeser modifications. (The old conservative position is now reactionary, and in practice isn't expressible in public anymore.)
The situation for gender is more extreme. The "bad" inequity positions have never been quashed, and since physical differences are more than skin deep, it has been harder to get people to agree that they don't matter. In the mass of different views it's a bit arbitrary to pick out a middle, but on one end might cite certain religiously derived views limiting women to the household, and on the other radical feminists who like to entertain the notion of parthenogenesis. There is a lot of room between these positions, and perhaps the center is to be found in the acceptance of the larger principle that employment should derive from actual ability and nothing else; that net differences between men and women should be accepted; and that the basic physical differences between the sexes prevent utter equality and that therefore some other standard of equity has to be proposed.
What strikes me about the difference between these "moderates" and those in Bushnell's examples is that the field of their "moderation" is different. Bushnell's "moderates" are compromisers; these moderates need not be.
That presents a problem with regards to same-sex marriage because there are really two questions involved. One is the metaphysical discussion of marriage, and this does indeed tend to bar a middle ground. The other, however, is the relationship between marriage and the state, and this is very much contested and allows plenty of room for a range of opinion. To help confuse matters further, in the USA this relationship requires compromise. One cannot keep "covenant" away from "law", not unless one is willing to completely bar the law from recognizing any kind of legal obligation arising out of a marriage. That would be an extremely radical position, of interest only to domineering, wife-dumping men. Thus, whatever position eventually prevails will be, on some level, a compromise.