There's a big difference in Sentimental Education between how much people think about sex and how much sex we actually see. And in a book about inaction, it's no surprise that we don't get much, um, action. But don't get us wrong: there's all sorts of partner swapping, sex triangles, prostitution, infidelity—you name it—going on. We get the idea that all of this sex is closely connected to power plays, just like almost everything else in the book.
Arnoux seems to really love Rosanette in his own weird way, but class-based social convention prevents him from ever marrying her; well, that and the fact that he's already married.
Frederick seems to have a habit of running away from sexual encounters (i.e., the brothel and Madame Arnoux). He might just be unable to handle the pressure.