There is a whole lot of drunkenness and sexual suggestion in this novel. Like, from the first page we're pretty sure that someone's going to get it on. Even the clouds are "tipsy." We get chapters and chapters of buildup with Bob trying to have sex with Lucy, talking about how much he wants to have sex with her, having erotic dreams, masturbating, and her talking about how aroused she is.
In other words, this is not a book for little kids. They should go read Hop on Pop or something. But we never actually get to see the sex scene that has taken up so much of the plot, and when it's over all of the sexual tension just deflates from the novel. For all of that, we'd normally give it a PG-13 since nothing happens, but there is that one scene where Bob goes all invisible and creepy on Lucy. Without her consent. So yeah. Pretty steamy.