Here's my theory: For several hundred years Christianity as led by white men has been dominant in this country - despite the fact that most of our founding fathers were Deists and they specifically did not envision America as a Christian nation. They are desperately fearful of losing that unearned privilege and for those they used to casually discriminate against to be considered "normal" citizens because in dominance hierarchy (like patriarchy) it's all zero sum and if you don't "win" you lose. They like being at the top of an artificial pyramid and they feel like the flattening of that hierarchy is taking away their rights. Clearly, it isn't but they believe in hierarchy and not egalitarianism. Plus whipping voters into a frenzy about this stuff helps get them elected.