Nudity because I just don't really care to see a strangers privates. I don't mind implied sex, as long as I don't have to see/hear it I'm fine with watching a movie with it. I won't watch comedies that have a lot of blatant sexual jokes or references. I don't mind some innuendos or hinting that isn't obviously sexual or dirty unless you think about it.

So in general I will not watch most PG-13 comedies because most seem to have sexual "humor". Depends on the PG-13 if it's a drama going by content and what's making it pg-13.