You'd have to talk to people who actually disagree, and I'm sure it's a completely bullshit reason. If I had my way, I would snap my fingers and all the religious right would simultaneously get over their stupidity and just marry gays and be okay with it.

But they won't, and the result is to create both an immediate problem (actual legal inequality between unions) and a longer-standing problem (a culture which is okay with this inequality and hesitant to accept gays). You can't change culture overnight, so trying to create a one-time fix doesn't do it. But you can fix the immediate problem of legal inequality right then and there, and this solution does so, while also providing a broader, much more conducive framework for gay activists to work with in order to bring about a longer-term cultural change.

I think the key misconception about this solution is that the proponents here are saying "Well, we just have to do this and we're done." I know better and I think the other proponents know better, too. But that's as far as a government can go policywise. Beyond that it's up to activists to use a much more accepting legal framework to combat the elements within our culture which resist the acceptance of gays.