Take things slowly would be my advice. It looks like there are a couple of issues here, I see that in the long run that your wife's feelings are very much at risk with this friend of yours. If he has a history of not treating women as people there's a good chance that she's going to end up hurt.
What is the issue that you have with your wife having sex with another man? If you can work though the reason why sometimes you can come to peace with it. After all you say that you don't have a problem with her having a sexual relationship with a woman.
It seems to me that she's not changing so much as discovering things about herself. Since she loves you she's coming to you and talking to you about it. Keep on talking about things and take baby steps. Neither of you have to get to the place that you're totally ok with a full blown second relationship overnight. It could be that she's chosen this guy as a first person to explore these feelings with because there isn't that expectation that it's going to be a long term thing with him. Figure out what you are comfortable with, the things that make you a little uncomfortable and the things you can't deal with right now. challenge yourself to try to push your boundaries on what you're a little uncomfortable with. Chances are that you will become more comfortable with things as they happen. Best of luck
Everything will be ok in the end. If it's not ok it's not the end.