Sentimental Education almost deserves two ratings: one for what people think about and want to do and one for what they actually do. In a book about inaction, it's no surprise that we don't see much, um, action.
There's a decent amount of partner swapping that goes on—especially with mistresses—but it's always behind closed doors. All we see is the longing, like in this scene between Frederick and Rosanette:
And, in a familiar fashion, she laid her finger on his chin. He trembled at the contact of her long hands, at the same time thin and soft. (2.8.15)
Ooh la la.
Anyway, what matters in this book is sexual desire—whether or not the deed gets done is kind of secondary.