First of all, if he truly loves his girlfriend and wants to be with her, he wouldn't be fooling around with you or anyone else. If he wants to explore his sexuality with other guys, he can but then he should be open with his girlfriend. If she isn't open to the idea of him being with other guys then that's just what it is. He either has to deal with that or find someone more open minded.
Honestly, he probably shouldn't be dating anyone at this point. The fact he couldn't adhere to the basic principles of a relationship just demonstrates how unworthy he is in being in one. Anyways, he's not being very considerate of you with this other guy you are interested in, even though you are also at fault in the matter. You should have denied his advances until he was single then had all the kinky sex you wanted lol. I'm sure if you remain friends with this guy, the truth will be found out. If that ever happens, just accept responsibility if you are ever confronted by his gf and don't make excuses. You wouldn't want it done to you so why would you think it would be okay to do to someone else. Also, you should not date this person. Not now, not until he actually discovers himself and what he wants.
If you want any sort of normal dating life with a healthy relationship then you should talk to your bi friend. If you want my advice on the matter, go back as friends. There should be no fooling around of the sort. No sex, nudes, ect. Get that friendship back and keep it there. If he doesn't agree or does and later tries to sway you then you need to ditch him. That's not a good friend then. He's just using you for his selfish needs and doesn't really care about your situation. Whatever happens, the boundaries have to be there or the string needs to be cut. This is because, any future relationships you have, if any of them ever ask you about your friend and if you hooked up before, or cheated and or was the "other woman", would you be honest or lie? What type of person do you want to be? If you are honest, they will most likely be okay with it because we have all messed around with friends before. Some work as far as maintaining a friendship and others don't. If you were to lie to them, then you clearly aren't ready to be in a relationship either. If you can't be open and honest with with yourself and your potential partner then just give up. Go back to being single and having your one night stands until the day comes when you are actually ready to form a relationship with another person. Remember, they aren't easy. You need to be trustworthy, able to communicate, able to take responsibility.
