Sexual arousal from fantasizing about having sex with someone other than your current partner. The thrill of a forbidden romance and the possibility of it turning into something real can be exciting. However, it is important to remember that fantasizing about another person does not in any way constitute cheating or cheating. It's just a private moment between you and your thoughts. So let yourself loose and explore these fantasies as much as you want!
By the way, have you ever asked your partner what infidelity means to him/her?