Yes, I think censorship boards basically send the movie back and say “as it is, we’ll have to give it an R because of that scene at 46.34 where they have sex and then she shoots the guy and his brains spill all over the floor. Cut that scene and we’ll give it a PGR”. So the producers make a choice to cut it and make more money.
This has nothing to do with the artistic value of a film. And a proper biography for example should show all aspects of someone’s life, including how they have sex, or take drugs, or behave violently.
But I think in most cases it’s the other way around: sex scenes are added (eg in a cop movie) to make the film more interesting, even though they are unrelated to the main plot of the movie. Why is it that every one who’s on the run from CIA finds the time to pick up a girlfriend and have sex with her while his stalkers are just outside the motel room? Personally when I was on the run I had my gf with me, but we didn’t get to have much sex until we were safely far, far away (which is where the movie would have ended). And the sex we did have (once, on a ferry) was in a cramped little bed which would not have made an interesting Hollywood scene.