Because showing sex scenes was taboo for so long, directors worked hard on dancing around the implications through clever dialogue delivery and suggestive cinematography. The result is that once they were actually allowed to do sex scenes, they'd built up this ridiculously huge, arcane bible of "how to shoot sex scenes and not get jailed for Communism" which basically precludes anything resembling actual human sexuality.
It's sort of like how explosions in Hollywood always have to involve huge fireballs and people flying through the air. The "cinematic" version has completely overtaken any sort of reasonable, natural depiction. The difference is that while most people don't see enough explosions to have a good instinct for what's wrong with a Hollywood explosion, most people have had enough clumsy, awkward sex to know what's wrong with a Hollywood sex scene.
It's sort of like how explosions in Hollywood always have to involve huge fireballs and people flying through the air. The "cinematic" version has completely overtaken any sort of reasonable, natural depiction. The difference is that while most people don't see enough explosions to have a good instinct for what's wrong with a Hollywood explosion, most people have had enough clumsy, awkward sex to know what's wrong with a Hollywood sex scene.