It basically depends on if anyone reports it. I have an AMG fic on ffnet rated M, with a mild explicit sex scene. It's a 7-year-old AMG fic, which no one ever reviews, so no one's noticed.

Harry Potter is far more noticeable. But if you rate the fic M, you're allowed an amount of leeway. provided you aren't having them expel fluids all over the place. :)