Differences of opinion on the issue of gay marriage in the U.S. are often explained away along religious lines: nonreligious folk support its legalization while the pious are against it. But that explanation glosses over some real differences in the way religious groups perceive the issue. As it turns out, certain religious groups show starkly contrasting levels of support for same-sex marriage, according to a report released today.

According to the Public Religion Research Institute, America is evenly divided on the issue of gay marriage: 47 percent favor its legalization while another 47 percent oppose it, as can be seen in the institute's chart above. But strong preferences manifest themselves among religious groups. Unsurprisingly, about seven in 10 of those who aren't affiliated with any religion or who are affiliated with one other than Christianity support same-sex marriage. What is surprising is that a slim majority of Catholics and a similar-size majority of white mainline Protestants support gay marriage too. In contrast, 60 percent of black Protestants and 76 percent of white evangelicals are against its legalization.

The explanations for these cleavages can likely be found in nonreligious factors. Catholics in the U.S., for example, are more likely to live in urbanized states than evangelicals, predisposing them to adopting more socially liberal ideologies. The support gap between black and white mainline Protestants might be attributed in part to differing cultural pressures within those racial communities. What's nevertheless strange is that these Christian groups, with a few exceptions, mostly take a hardline stance against gay marriage, yet followers adhere to those stances at vastly different rates. But these religious differences may not matter so much in several generations: the main theme of the study was that younger people are supporting gay rights at much higher rates than their elders. It found "at least a 20-point generation gap" between 18 to 29 year olds and adults over 65 on every public policy issue concerning gay rights. And seven in 10 people in that younger age bracket say that religious groups that come out against homosexuality are alienating them.