Anything that covered anything to do with sex or masterbation. UGGHHH!!! I hated those studies. I remember a few years ago there was a Watchtower article on sex and oral sex came up in one of the paragraphs. The elder doing the study is this really oldschool guy so he asked the question, "Just to be absolutely clear, is oral sex permitted by bible standards?" Crickets. Then this old crotchety woman, the kind that hasn't had sex since Russell was alive, raises her hand and boldly condemns the "immoral practice." All the young people were looking at thier feet.
I think that was the most interesting watchtower I've studied, now that I think of it.