Is it normal to want to watch my girlfriend with another man?

Is it normal to want to watch your girlfriend with another man?
Basicly my girlfriend and I went through a rough patch last year, I left her as we were having some real comunication issues and I allowed my work to consume my life because of a death in the family. So we started seeing other people after a few months, we then decided to get back together after a long chat, so we get back together and few months in she admits to cheating on me. We dealt with it, (I was obviously feeling betrayed) but I believe I trully love her enough to forgive her and overlook it. (I've never cheated on her) and the guy she slept with was a former friend and co worker of mine, (we both work in radio and tv, but he has a past in the adult film industry) so she slept with him a few times, and he is pretty much a pornstar, and now, months later when I masturbate or when we have sex, I imagine how their sex was and what it was like for my girlfriend to wrap her lips round another guys cock... This has become a regular fantasy, and I want to watch her have sex with another man. It actively gets me off. I haven't told her my fantasy, but is this normal? After your girlfriend has cheated on you to want to see what she looks like with another guy?