Why no sex in movies? (Usually)

I often say I want movies and TV with sweet nudity without sex. And that's true. But also true is that I find it very artificial that there is a strong separation between movies and porn.
There are the very rare examples which reach towards the other. For example, multi-award-winning movie Teenage Lesbian from Adult Time. That's porn, but also a movie, and a good one actually.

And from the world of cinema is the also multi-award-winning movie Blue Is The Warmest Color. It's a poignant story and it also has a couple of really flaming sex scenes, and one of them is five minutes long. And while there are no closeups of course, they really don't hide that they are actually doing it. (Both movies are lesbian, fortunately.)

I guess the problem is that sex is so strong a mental influence that it tends to take over the mind so you can't focus on the story. But I've pretty much gotten over that, and I have noticed that I really don't find a disconnect between scenes with good story-telling and scenes with explicit sex, it can live together. 

And I hope for many more movies in the past which bridge these two worlds.