At least for the vast majority, that is. See here:
With no debate, Republicans at the party’s spring meeting here on Friday unanimously approved a number of resolutions, including one that reaffirmed the party’s opposition to same-sex marriage.
“The Republican National Committee affirms its support for marriage as the union of one man and one woman, and as the optimum environment in which to raise healthy children for the future of America,” the resolution read. The 157 RNC members present approved it in a voice vote.
I’ve made the claim in the past that the reason people turn to the GOP is out of their conservative Christianity. To me, this is a very tiny, very obvious claim. Basically dishonest people who aren’t interested in critical thinking (or doing any research, but I digress), such as the odious Michael Hartwell, have tried to spin my statement in a way where in order to prove it I would have to explicitly know the minds of every single Republican. Under his requirements, we could never surmise why anyone becomes anything if the group we’re discussing is sufficiently large. (This is interesting, too, since he has gone the racist route of claiming that blacks vote for Democrats because they benefit from and like handouts.)
At any rate, I think this is all quite obvious: Most people who become Republican are first fundamentally religious, soon recognizing that there is a political party which reflects their religiosity. The re-affirmation of the GOP’s opposite to marriage equality is a perfect example of this because there are no good (or even honest) secular arguments against allowing same-sex couples their constitutionally guaranteed right to marriage. That is, it is the base Christianity that underlies the Republican party that has caused this vote and view; we don’t live in some backwards world where people became bigoted Republicans all on their own, later noticing that a particular cultural religion happens to exactly reflect their positions.