Once upon a time movies were so subject to censorship that rude words and any hint of sexual activity were impossible to put on screen. Directors could hint, but couldn't show. Go back and watch Humphrey Bogart and Lauren Bacall in the 1946 version of The Big Sleep where plenty is hinted at but nothing much happens.
Now we have censors in some countries relaxed enough to allow real sex to be shown on screen in non-porn movies for mainstream audiences (see Do they really have sex in movies? for discussion of the example Nine Songs).
So what was the first mainstream (non-porn) movie where something like a real sex scene (simulated or not) was shown?