It's different. One big part of the reason has already been said in sibling comments: taboo. Kids know that the huge jumps in martial arts movies are impossible because they jump when they play, they have seen their friends and classmates jump, they probably have tried flying kicks when playing so they get an idea of where the limits are. Nothing of this happens with sex, plus often they aren't exposed to anyone talking about it, except of course in porn.

The other part is the huge insecurities people have in this domain. You will meet a lot of people who aren't afraid to tell you that they dance like crap, or have no musical ear, or are in bad shape, etc.; but even if you meet people who talk about sex, no one is going to tell you that they last one minute in bed.

Taboo exactly; I will freely talk with colleagues and friends about e.g. GTA or action movies; watching movies and playing games is a communal and public thing.

But nobody talks about what they do in the bedroom; nobody goes to a movie theater to watch porn, people are awkward when there's sex scenes in films (and mainstream films have stopped having them altogether it seems), teenagers run away if their parents ever broach the subject, etcetera.