you write people having good sex you’re just writing porn but you write people having bad sex. now you’re writing a story
and if you write bad sex intentionally as part of A Story, people will take quotes of it out of context and share them around the internet and laugh at how unsexy they are and therefore how you are a bad writer and [people of your demographic] should never ever write about sex!
(This has never happened to me. I am an amazing writer who also only writes porn, so all my sex scenes are devastatingly arousing. I’ve just also seen a lot of “bad sex scene” quotes shared and laughed at without context when the only thing that makes them “bad writing” if is you’re assuming the author’s intention was to get you horny.)