Rep. Charlie Dent's support of gay marriage is not the usual Republican stance on the issue. (AP Photo/J. Scott Applewhite)

This week featured a major event in the Republican Party's still-ongoing evolution on gay marriage, with Rep. Charlie Dent (R-Pa.) becoming the latest of a handful of congressional Republicans to say he supports it.

But while Democrats have come out en masse for gay marriage in the last couple of years, don't expect a similar onslaught from the GOP any time soon.

The reason? The GOP is still struggling with the morality issue.

While Democrats have moved quickly away from the idea that gay and lesbian relations are morally wrong, Republicans are moving much more slowly. In fact, they're barely more accepting of homosexuality today than they were a decade ago, according to Gallup, which has been asking Americans every year whether various behaviors are "morally acceptable" to them.

(Chart based on Gallup data)

While the GOP has taken a turn for the more tolerant over the last few years, it's actually only slightly more accepting of homosexuality today (39 percent) than it was in 2005 and 2006 (36 percent). That difference is within the margin of error.

Over that same span, Democrats have become 20 points more accepting of homosexuality, rising from 51 percent acceptance to 71 percent acceptance.

So while there was a 15-point gap between the two parties in 2005, it's now twice as big, a 32-point gap.

The morality issue is a key one for Republicans. While it's possible that Republicans who think homosexuality is morally wrong could still back gay marriage, it's much, much more unlikely that they will ever get to that point. Morality is, after all, the prevailing reason that people give for opposing gay marriage. If you think homosexuality is wrong, period, then you'd have to be pretty libertarian to think gay marriage should be okay.

For now, Republicans who want to expand the party's appeal to the gay community and avoid being on the wrong side of a fast-moving social issue will likely simply say that it's an issue that should be left to the states. We've already seen this approach from many leading GOP politicians and would expect it to continue to creep into play during the 2014 and 2016 elections.

As for Republicans jumping on-board with support for gay marriage, though, it's still a very difficult proposition.