To me, part of why the movie Boyhood rated R is because there is a lot of discussion about sex and some mild nudity in this movie.
When the characters discuss sex it can be rather explicit, and there are times when young boys are looking at lingerie catalogs and porn online while making inappropriate comments. At one point a naked man and woman are sharing a bed, and although they’re mostly covered up by a sheet it’s implied that they were having extra-marital sex. Another scene shows a dad talking to his daughter about using condoms for protection, but it’s pretty realistic, even though it’s humorous.