In the wake of the controversy surrounding the much maligned "religious freedom" legislation in Arkansas and Indiana, Gallup aggregated data it collected between 2011 and 2014. The conclusion is straightforward: "religiosity is a strong predictor of attitudes about same-sex relationships in the U.S. today."
Gallup used responses from more than 4,500 interviews conducted for its annual May Values and Beliefs survey, in which the pollster lists several items about which interviewees are asked to rate as morally acceptable or morally unacceptable. As it turns out, the more a person goes to church, the more likely that person is to consider homosexual relations morally unacceptable:
Considering that many Christians believe homosexuality is a sin because of the prohibition against it in the Old Testament, these figures will surprise no one. Yet to see these gradations of intolerance so neatly correlated with religiosity should throw cold water on the notion religions aren't intolerant, only people are. Of course, there are plenty of religious people who endorse marriage equality, but doing so means either ignoring outright the anti-gay preachments of their holy text, or employing an interpretation that is fundamentally at odds with most religious teaching on the subject.
Such anti-gay sentiment is what happens when human beings continue to get their moral standards from a book that was written before humans knew basic truths like disease being caused by germs or the sun being at the center of the solar system. Consensual homosexual relations do not do any social more harm than consensual heterosexual relations do. Thus, the proscription against homosexuality in the Bible is a classic case of malum prohibitum. Homosexuality isn't wrong in itself (malum in se), but is only regarded as such because god said so.
Unfortunately, casting aside the antiquated "wisdom" codified by relatively ill-informed ancients is much easier said than done, as those familiar with Americans' preference for creationism over Darwinian evolution can attest. According to a Gallup poll conducted last year, 42% of Americans believe that god created humans in their present form. Another 31% believe that humans evolved but with god's guidance, and 19% said humans evolved with god having no part in the process:
The physical evidence for evolution, of course, is immense. And yet, more than 40% of the country deems this insufficient for the plain the fact that it would contradict a literalist interpretation of the Bible, which as the "word of god," must be treated as such as far as they're concerned.
While 58% of Americans regard LGBT relations as morally acceptable compared to 38% who don't, that 38% likely consists largely of fundamentalist Christians who will not simply change their minds to get on "the right side of history." For them, they want to be on the right side of god, even if it means it means damning history and an entire group of people simply because of who they are.