I think there are two trends. 1) in the early 90s people still got their porn fix from magazine stashes hidden in the woods. Or so the legend goes. So a sex scene or just some boobs were a welcomed element in a movie. In the 70s it was quite common to have absolutely gratuitous female nudity in movies that didn't need it. Fast forward to the new millennium and you can get any porn you like on your phone in 15 seconds. People don't go mad if there isn't a sex scene in it any more. Sensibilities have changed; also in regards to what actors -however well paid- have to go through. Also, US movies have always been more tolerant of violence than sex.

2. Movie financing has shifted more to China and they are even more prudish than the Americans as far as movie censorship is concerned. If you want your movie to get a license to play over there, don't make the Chinese the bad guys, don't kill any Chinese people in the movie, and switch to footage of a train going into a tunnel when it gets hot and steamy.