My girlfriends ex

I have this confession to make:
I love my girlfriend and I adore her mind and body (she looks like this ideal metal-gothic girl with black hair and stunning blue eyes and short black dresses). But I fantasize about her having sex with her ex. How this fantasy came along happened as follows.

When we began to date we used to spend hours in bed talking about everything. Me still being a sexual beginner (although very interested and well-read), I deepened my questions on her former sexual adventures:

She told me about her having sex in the midst of a festival. Her having sex on a graveyard. Her having a threesome with her best friend and her ex boyfriend. Her having sex with her ex at a party to show her ex ex what he lost having been unfaithful...

So one time we went to a party with her old boys and I couldn't get my eyes of her sexy skirt (and being in the just fell-in-love-phase). She was gone talking with her ex for what seemed an eternity and I got drunk because of my jealousy. I went to a closed room where I found both of them hugging.

Later it turned out that she was just apologizing for her way of ending their relationship and I totally trusted her. But in the moment I was shocked and jealous, but at the same time I couldn't get the what-if-scenario and all the other stories she told me out of my head. I was so turned on. Now all the sexual adventures had a face I could connect them with.

It seems that my girlfriend has already past her phase for sexual adventures and experiments so I can't help that this kind of fantasies are getting stronger and stronger.