My girlfriend is infatuated with another guy?

I have had an on/off relationship with my girlfriend for 8 years. I've always been attracted to her, but we didn't seem to be compatible. We had not been involved with each other romantically for about 3 years, until about 6 months ago when I decided to give it a serious try. We've been getting along much better than in the past, and we do things together almost every night. About a year ago (when we weren't involved romantically) she had mentioned another guy she had met in ping, and joked that she wanted to give him a BJ and get married to him. She has wanted to get married since I met her, but she dresses slutty and she is very aggressive ually when she meets a man she thinks is a good candidate (money seems to be the major factor, and she practically drops her pants at the sight of a very wealthy man). I know she had come on to this guy very strongly about a year ago, but I highly doubt that it went anywhere. He's out of her league, and she has no real chance of anything other than perhaps a ual fling in which she gets used by yet another man. She has kept in touch with him, and I can tell from the way she talks about him that she is very strongly attracted to him (she claims to not be interested, that he is just a friend, but that's clearly BS). He lives in another city, and I am almost positive she has not cheated with him. Given the amount of time I spend with her, there's really no way she could be talking or texting him frequently without me knowing about it. But, she recently said she wants to travel to the city to attend a social event of some sort with with this guy. About 4 months ago, she traveled the other city or a similar social event. On the drive back (not an overnight) when I was talking to her on the phone, she made a comment about how much she really wanted to have with him (I'll leave out the vulgar expression she used). I got mad, and she ped it off as "joking", but she doesn't "joke" like that about any other men. On the one hand, I don't want to be insecure and controlling, but I know her quite well and I'm fairly sure she will try to get this guy in bed in hopes she can entice him into a relationship (she leads with her crotch, and he is a VERY big "fish" for a gold digger). The odds of her succeeding are low, but I'm not wild about the situation. I'm pretty sure she wouldn't go if I make a fuss about it, but she'd hold that against me. I realize that in reality almost every long term relationship involves "settling" to some degree, and people will always be attracted to others outside the relationship. But, it bothers me that she is trying to keep her other options open for this long. (she has been pressuring me for several months now to get married, and she talks to all her friends about it and says she wants it to last forever) Am I just being insecure and jealous? Should I just let her go and let the cards fall where they may?