Difficult situation with girlfriend and ex(s). stick around?

The situation is rather peculiar but I desparetely need a second opinion!!! So I will appreciate any comments.

My girlfriend used to be sexually involved with this guy who supported her psychologically after she broke up with her cheating previous boyfriend. They stayed sexually involved for a short period of time but they remained friends. Later on she introduced him to her best friend as a good guy to date.

Later on I came into the picture and now I am stuck in a situation where this particular dude is dating her best friend,... and there were a few occasions where my girlfriend asked me to go out all together as couples. He seems a good guy but obviously the situation sucks for the male brain.

Just yesterday I found out that after she stopped being sexually involved with her friend, she had sex with her previous cheating boyfriend (and a guy as she says that made her life a living hell). Now please take into consideration that she still speaks once in a while with most of her ex boyfriends.

So first of all what do you think of the situation?
Should I stick around? What would you say about her personality?

By the way.... I need to add that I told her that I have no problem if she goes out with her best friend and her boyfriend but I wouldn't feel comfortable because the guy is her ex sex buddy.... which to that she replied that I am "passive aggressive".... Am I?