The text they rally behind as a fundamental part of their religion, in no uncertain terms, promotes violence against gay men and tells you women are worth a fraction of men and can't be trusted to preach. Not to mention the endorsement and regulation of slavery.
It's not that they're a monolith of bigotry or anything, it's that they start from a pretty messed up place and have to mould that out of their understanding of their religion, and plenty of them don't.
But the real issue is that you can justify just about any sort of prejudice when that is your foundation. There's no shortage of Christians who cite Leviticus to tell me my sexuality is an abomination, yet they dismiss the parts about slavery because "that's the old testament." The Bible also doesn't say anything about trans people and it doesn't oppose abortion rights, yet the majority of the Christians in my state are opposed to both.