GTO is as realistic a manga as they get and so....
Sexual situations/references/jokes are abound.
I read up to manga volume..... 15? <-- again doesn't remember.
But really, the stuff in there is NOT for kids, rated R material though not as "hentai-ish" as Golden Boy.
It's hecka funny if you get all the jokes (references from popular games, anime and even The Ring) and the details are also great. Story-telling is all very real since it IS a highschool drama--somewhat. Also the usual "moral messages" apply: never give up; friendship; be yourself; etc.
But really, R rated for mature situations and LANGUAGE (I nearly forgot that). Very very crude language (not only cussing but also when talking about sex, etc).
Surprisingly, no explicit nudity, but more than enough dirty stuff to ensure a good thrashing from the Holy Spirit
And 'some' violence too ^^;;;
(guiltily, GTO is actually a fav manga of mine ^^;;; )