Gmans uncle said:
3) Straight men disproportionately govern what we see in the main stream. These men think that women, gay/bi men do not have a strong response to male nudity and likely themselves find it pretty damn funny and think everyone else does as well, so they don't think twice about including it where female nudity would not be allowed.
If I may jam my filthy oar in to add another perspective to this final point; main stream media is disproportionately governed by men, AND disproportionately affected by traditionalist and/or reactionary people from the religious/right wing (as Katatori-kun suggested simply because these are the kind of people more likely to complain and thus have a disproportionately large voice in these matters) who still view the female form as any and all of these things; shameful to be sexualised, shameful to be displayed in any form, only capable of being sexual and nothing else, the 'innocent virginal special flower' type mindset where 'the fairer sex' must be protected from exploitation due to their weak vulnerability, or any other, similar, attitudes I've missed out.
The differences, then, are in the way society (or, at least, THESE people's society) portrays the different genders' bodies. Male is powerful and proud (which, in the age of cynicism has translated to humorous), and thus able to be portrayed in many different ways, whether sexual or not. Female is fragile, beautiful and intimately sexual, and thus can only be portrayed in such a way.
May I also add that all of these are trends that I would very much like to see the end of (and not just because it might make female nudity more acceptable wink-wink) but because I think it's unfair for there to be such a double-standard. In a broader societal context, I know many girls and just about all of them would love to be able to go completely shirtless in public like males can if it's hot or just because tops and bras are uncomfortable.
I have to say I agree with Gorfias, and this is a trend which I first noticed in the movie 'Eurotrip' (a movie I really like btw) where there's an entire beach full of naked men showing full frontal nudity with everything hanging out for long enough to get more of a look than I (as a heterosexual male) ever want to have again. This is at a nude beach, mentioned at least twice in that section, and when they give a shot of the female part of this famous nude beach containing 'Europe's most nubile exhibitionists' they're all topless with underwear on with not one exception. The reason I find this so jarring, and more indicative of a trend than any single movie really should be, is how much they play up that it'll be a nude beach filled with exhibitionist females, and then even in the 'unrated' version they shirk showing proper nudity, which they blatantly did for males, clearly opening up a plot hole and breaking the element they've gone so far to characterise for this sole reason.
Anyway, as per usual, my post has ended up roughly 8 times as large as I originally intended, so that's my two cents and congratulations to everybody who read all the way to here. To those TL;DR, don't worry I probably would have done the same