Well there are certain things that mark a lack of knowledge of gay sex for sure - lack of lube or use of something as lube that is NOT advised irl are a biggie there. Like even someone who's never had anal sex or penetrative sex should generally be able to figure that one out, but whatever.

I personally am not super sexually active to have the breadth of experience to know for sure, but I reaaaally wonder sometimes about described sexual positions that seem particularly bendy or impossible. And amounts of cum that are possible or how quickly a guy can get it back up. Just based on conversations with other people about sex and talking to guy friends, I suspect many fic writers are pretty okay with being nonrealistic about those things.