Female Leads in Smut Manga

I’ve read lots of manga with smut and I’ve come to realize something very jarring. Why does the FL always act like she dislikes sex? Screaming things like, “Stop! It’s embarrassing! Don’t look at me!”. Not to mention if you look at the covers of said smut the FL always has a fearful look on her face. You’d think she was being sexual assaulted, but when the act is over, she has a happy look on her face. If the FL doesn’t seem into it, how are we, the reader, supposed to be? And the FL never wants the ML seeing her naked despite them having sex multiple times.