I vote Democrat because I can't stand the Republican party being all caught up with the Christian right wing.
Social conservatism as dictated by Christianity is patently against everything this country is about. Hell, it's even against everything the GOP is about. The underlying Republican philosophy is that our federal government should be made smaller and less intrusive, and that in general laws should not be made to impose on what citizens can be done in their own homes. However when it comes down to an issue that Christianity doesn't like, the GOP goes and decides to ban things on any level it feels necessary. Case in point, the desire for a Constitutional amendment to ban gay marriage. That's exactly the opposite of keeping the federal government out of our private lives, but since Christianity has a position on the matter the GOP goes back into bible thumper mode.
I'm probably more economically conservative and more socially liberal than most Democrats. While some (and by some I mean a very small quantity) Republican economic ideas make sense to me, I feel so strongly that the church does not have a place in determining social policy that I will never even consider voting Republican until they end their affiliation with the church.