Okay, so there's no actual sex in Marcelo in the Real World. But there's all kinds of thinking about it. Wendell wants it constantly, Jasmine doesn't get why everybody wants it with her, and Marcelo's just trying to figure out what all the fuss is about. The obsession with sex is just one of the facets of the "real world" that baffles him completely. "Sexual intercourse," as he calls it, is nothing more than how babies are made, right? And if two people have sex, doesn't it mean they're in love? After all, that's how he's always been told it works. Ah, if only it were that simple.
Marcelo may not know about rape (we never find out for sure if he does or not), but he instinctively knows that what Wendell wants to do with Jasmine isn't right.
Arturo and Aurora don't appear to have talked to Marcelo about sex, like most parents do with their children. It appears that they've relied on Paterson to teach him.