Firstly, when you say that he’s fantasizing about her, does he do that intentionally or not??? I know that she is the one that presses the “start” button, but are these fantasies like some things they stick on your mind and you can’t control or you can choose and not think about them. The way I saw it, at least up to now, was that she triggers something to his mind/body and then he does what he “has” to do. Is it this, or he tries on purpose to stay like this, just to masturbate later on thinking about her.
Secondly, in order for a guy to stay with a hardon for more than an hour how positive are you that he was fantasizing having sex with her? Also, if he did fantasize having sex with her was it a single thought or he keeps fantasizing about her for hours?