Personally I think that this is a result of a cultural readjustment. Consider that prior to the sexual revolution, most male nudity was seen as not really a big deal. I think that it was a response to the counter-culture movements that created a response to that counter-culture (The catholic church did a similar thing in music back during the protestant reformation). In truth I think that the male nude, hailed as the defining artistic tradition, is simply coming back into what was already there. The female nude, however, exists because of the prevailing culture war. Consider that conservatives are the ones who desired to "protect" women's modesty. It doesn't make much sense for this same culture to do a 180 and say that naked women ok, naked men no goes. That is unless that culture is seeking to fend off some other culture, which I think explains this problem quite well. It also doesn't help that some people are demonizing masculinity to counter-balance the perceived "patriarchy". Regardless of your stance on said issues, the political-cultural forces are going to take a while to settle on this issue.