Willie Stark is married and has at least three girlfriends. We actually never see any of the alleged sex, but there is so much talk about it that we get the point. Jack on the other hand doesn't have much sex at all. Back when he and Anne had a budding romance, Jack decided that he couldn't sleep with her, which basically ends their relationship. Jack does seem to have a good sex life with Lois, but when he realizes she's a person, he decides that he'd rather not have sex if it can't be with Anne.
Sex has a really bleak side in this book through the story of the affair between Cass Mastern and Annabelle Trice. This sex is bleak because it represents a betrayal of Duncan Trice. Later, Annabelle even sells her slave, Phebe, into sexual bondage.