Hello everyone, I just Googled a similar question and am glad I found this forum

I'm currently going through a similar situation, only difference is that she introduced me to this guy friend while we were dating and I thought they were just friends and only friends, I found out later that they had sex once, she never lied to me about it. My girlfriend has this guy friend, whom she has known for 5-7 years, and she claims to only of had one sexual experience with this person, let's call this guy Douche. She tells me that her and Douche even kinda dated for like one week and then she had to put him back in the friend zone where he's been ever since.
After all these years, Douche still likes my girlfriend, I know this because I've seen how he's all over her Facebook page, desperately "liking" and commenting on everything she posts and I also know this because she told me that she knows he still likes her. But, in the friend zone he has allegedly remained and I do believe her btw. She has had several boyfriends since starting up a friendship with Douche and when I asked how serious their friendship is, she said it goes in "sperts." I asked how her other boyfriends handled the situation and she said something along the lines of that's when the friendship wasn't as close. I assume she runs to Douche's company when she's not in a serious relationship because they like to party and go to Raves together. A respectable man, who's not a Douche, would say something along the lines of "Yeah lady, I notice you drop our friendship when you get with someone and come running back to me when your feeling down or single? I'm sorry but I don't do back burner friendships." Obviously Douche has never come close to gaining some semblance of this type of respectability.
Since we have gotten together, out of respect for me and what makes me feel comfortable, she has avoided hanging out with Douche, of course Douche is mad about this and feels that it's because of insecurity that a friendship of 5-7 years is no more. This is expected from a Douche who will happily be confined into the friend zone and will wait and hope for the opportunity to catch a vulnerable rebound, or that she may "one day change her mind and see how much of a great guy I am." I know guys like this and it's sad that some men have to resort to such tactics and have literally no shame.
"What's good for the goose, is good for the gander." My girlfriend agree's with that statement and has admitted that she wouldn't be comfortable if the roles were reversed. It pisses me off that she couldn't of figured this obvious $#%^ out before I had to mention it and practically spoon feed it to her, but whatever, I'll gladly take that we see eye to eye on it now.
It's simply NOT appropriate people, period!! When I have a relationship, I think of this woman as potential marriage material and I cannot imagine my future wife wanting to continue to hang out with, and be friends with, another man whom she "gave in" to a sexual experience with before we met and especially knowing that this man, if given the opportunity, wouldn't hesitate to pounce. It would be inappropriate for me to do the same thing to my future wife or current girlfriend.