Kei Kurono from Gantz. At the start of the anime, he's well-known in his school for jour dreaming about girls, then getting a boner in class. He's a pretty good example. At one point he has sex with this girl he doesn't know so well, in order to try and make this other girl jealous, because he likes her, but she actually likes his friend, which makes him jealous. Lol.
