I'll try to think of a better answer.
But, the operative word in your question was "sexual" not "romantic".
One scene that comes to mind is in The Terminator, but it was a rather dark shot.
It's a pivotal scene though because we come to learn that this is when John Connor is conceived.
videocelebs.net/...n-nude-the-terminator-1984.html
If I recall correctly, we don't actually see the sex, but it is implied because it is the central theme of the movie...
About 50 years ago, there was an important movie called "The Summer of '42", in which a teen boy, Hermie, about 15, who, like many teens, does not have much sexual experience, but, as a boy, is horny all the time. That summer, he is vacationing with his friends. He becomes friendly with a gorgeous married woman, Dorothy, played by model Jennifer O'Neill, whose husband is off fighting WW2. Of course, the boy is infatuated with her and is like her puppy. One day, she gets the telegram from Uncle Sam. We know what it says from her response - her husband was killed in the war. Upset, she finds solace in making love with the boy, but then she disappears never to be seen again by him.
https://www.youtube.com/watch?v=HK-AgsP0zfY
Most of the Dreamers. But definitely when Eva Green's character goes to give him a blowjob and finds a naked picture of herself on his dick.
The Blue Lagoon scenes are pretty wonderful.
I found the Halle Berry scene hard to watch
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