I'm not a movie person, but after re-visiting a lot of the big movies I saw as a little kid in the '80s, I'm struck by how much full or partial nudity there was compared to now. My hunch is that this reflects the general transition during the early-mid-'90s from wild times to tame times. It would take more effort to find out just when the "nudity in film" change happened, but as a brief comparison, consider just 1984 vs. 2009. We only want to include movies that lots of people see; if few see it, it's no big deal.
I looked up the top ten grossing films for both years at Wikipedia and then went to the "Parents' guide" section on each one's IMDb page. This lists instances of full or partial nudity, other sexual content, violence, profanity, and so on. It seems to be edited by paranoid parents who labor to tar all movies as depraved, so if there was something serious in it, they wouldn't miss it. So how do the hit movies of 1984 differ from those of 2009?
Each year had a hit movie that showed a non-human female with a mostly-nude costume that showed some of her breasts -- Ghostbusters and Avatar. I don't think that counts, but it's hard to call. For true cases of showing bare breasts, bare buttocks, full frontal nudity, etc., 2009 had only one hit movie with such images (The Hangover). However, five hit movies of 1984 did (Beverly Hills Cop, Police Academy, Footloose, Splash, and Purple Rain). So, the typical high-exposure movie from 1984 would have shown something, and the typical one from 2009 would not.
Perhaps the clearest illustration of this shift is in the amount of nudity shown in screwball teen movies -- if any genre is going to show something, it's this one. In Porky's (1981) there's full frontal and backal nudity, other shots of bare breasts, and so on. In Dazed and Confused (1993), there's nothing. American Pie (1999) has only one sequence showing bare breasts. And when we reach Superbad (2007), they don't show anything at all. It's understandable if they don't show anything in teen movies that aren't about slackers and adolescents trying to get laid, like Ferris Bueller's Day Off or Mean Girls. But even the screwball teen movies from at least 1993 onward refrain from showing nudity. And conversely, even a general coming-of-age movie like Fast Times at Ridgemont High (1982) had full frontal nudity back during wild times.
Because the movie industry is one of the most competitive out there, it must be that this shift is due to changes in audience demand. If young people today still wanted to see bare boobs and butts, the movie studios would give it to them. This shift in their cultural tastes parallels their behavioral changes; promiscuity among high schoolers has been falling since 1991. Therefore, it is not simply a substitution of internet porn for teen movie nudity. It's part of a larger pattern.
(Also, adults weren't going to see Beverly Hills Cop or The Terminator in order to see T&A, since they had easy access to porn. Nudity in hit movies is not there to physically arouse the audience but to suggest wildness in the environment.)
Another curious thing I notice that's related is how juvenile young people want to remain today compared to during the '60s through the '80s. During all times, teenagers want to become independent and start living what they picture as the cool young adult life. Still, there are pendulum swings around that constant desire. Just look at how popular Ice Age, Harry Potter, and Transformers are. Or Pokemon -- still. Flash back to Fast Times at Ridgemont High again, and teenagers are worried about getting the right job in the most high-status part of the mall, or moving up from a job at a crummy vs. a respectable fast food place.
When young people perceive the world as pretty safe, they're going to delay going through rites of passage -- omigod, why should i hurry? i mean it's not like the world's gonna end or anything. Of course, when they do believe that life is shorter and more dangerous -- such as when the violent and property crime rates are going up -- then their mindset is more one of "piss or get off the pot." In particular, it's time to get a job and to work toward making some babies. While tame times last, though, they're more interested in meta-ironic-detached portrayals of rites of passage or else childlike fantasies. In the '80s, it was mostly little kids who were into The Neverending Story -- not college students, who were too busy working, driving around in their cars, drinking or doing drugs, and getting it on.