If you were to look for SSM opponents in 50 years, they'd mostly be traditionalist Catholics, Reformed Protestants, fundamentalists, and some Orthodox Jews & Muslims.
I largely agree with DC, but with the caveat that the term "traditionalist Catholic" does not mean a Catholic who believes in the Church's teachings on morals. It refers to Catholics who have, for whatever reason, rejected the Second Vatican Council and often form schismatic fringe groups (though not all trads are in a state of schism). While I agree the traditionalist Catholics are incredibly unlike to support gay marriage in 50 years, they will almost certainly be a very small minority within the subset of Catholics who will still oppose gay marriage.
Also, in 50 years I expect this issue to largely be depoliticized with most of these groups effectively accepting that the government has a different standard for recognition of unions than religious organizations. I think if I were to make one criticism of the religious gay marriage opposition movement at this point, it's that we're often more fixed on what the government does than we are about teaching sexual morals to be followed in our private lives. I think to a certain extent the focus on this issue occurred because until about five years ago it was an easy one to preach about. It didn't affect the lives of most people. Whereas, homosexual sex acts aren't all that morally different from say masturbation, which I doubt receives as much coverage. Interestingly, until about a year ago, I never heard homosexuality even mentioned in a Catholic church from the pulpit. I was somewhat haphazardly catechized on the topic in CCD class, but it was like a five minute footnote. I think we can all agree the current strategy of opposition isn't really working. We need to make it less of a political issue and more of an issue of personal piety within churches. I expect in 50 years that's what will happen.
There is a big problem for the Catholic Church in issues of sexual morality and that is being essentially a ‘sexless’ body. Even if we are to think well of Catholic priests, bishops etc (and that is, and I am not being flippant here at all, an increasingly difficult thing to now do) on what basis can they talk about such matters? How can a priest talk of sex when he can’t have it, marriage when he can’t marry, family planning when he can’t have his own family and children when he will never have them? On what basis can a sexless 60 year old male virgin make a claim on what is sexually right moreso than say, a parishioner? Now I could give you my opinions on fishing and they can be considered as part of a reasoned debate on that subject. Ultimately however, I don’t fish. Even a person who fishes badly knows more than I do about fishing. So when priests talks of say gay relationships, not only do they not know the intricacies of the emotional bond between gay couples, they have no idea about the emotional bond between straight couples with which to make any comparison between the two. That is why they are finding themselves increasingly and welcomingly sidelined.
