I'm so sad for you; wish I could give you a hug :( I can't imagine how shocking and hurtful that must have been to find out. That the person you so adore is just NOT who you thought he was. I really feel for you; kind of wondering about what happened next though. Did you end the relationship or... did he leave the other relationship?
I guess this is really one of those hard situations where no matter how broken you might feel by this, you HAVE to put yourself in his position and think that perhaps he is terrified about revealing the truth to people he knows. Or maybe there are people in his life who would completely abandon him or become hostile or dangerous. Can't really ever know I suppose... Is he at least being supportive towards you after you've suffered this hurt and shock? If not, probably would be best to just take some time away and pick yourself back up. If you really do love each other though and both of you want to continue, I guess you will have to accept that this has happened and maybe try to support him until he figures out how to be open with everyone about what he really wants?
Good luck to you sweetie - I hope you'll update everyone on what has happened.
I'm so sorry that this is happening to you, and the girl in question. For whatever the reason, even if my partner cut if off with the other person, i wouldn't be able to trust that they had the moral aptitude to take up more of my precious life. You cannot find Mr. Right while you're in this three way situation , nor can Mr. Right find you. Have faith that there is someone else out there that would never cheat. Go forward with eyes wide open and be more conscious of time apart - have a reasonable expectation of when things get serious enough to move in together for example. Don't do years and years of solo living not knowing what another does with their days and evenings.
I think it is not really so unlikely that the reason he is with her is to mask the fact that he likes guys. Or, at least, that this is the main reason. Men in some positions in more conservative worlds (more conservative than the West and East coast of the U.S. and parts of Europe) do feel obligated to hide being gay or bisexual, and/or feel conflicted enough about it that they try to just go along with society expects in order to see if perhaps it will make them see the light and get straight. If he is not much past age 20 or 21 and lives in a conservative world, he might be telling you the truth.
In your shoes, I'd try to set aside the part about her being female, and just react the way you would if you found out that he had a male partner who he had been dating for two years, but now tells you he really doesn't care about that much. I'd ask if he cares about you more, and ask what he intends to do, because you don't want to be in a relationship with someone that is seeing more than one person. (If that's so.) Tell him it hurts your heart, and you don't want to be in it knowing it will only hurt you more. See what he says.
That's really hard. I feel bad for you AND him as he lives a secret life. But please don't believe 'the only reason' he is with her is because people don't know he likes guys. That's not true. He'd be alone if that were the case, right? He's IN a relationship because he wants to be and certainly has some feelings for her. Bisexual men do find themselves in this position.
When someone cheats, that is always hard. Hard on you, hard on the person they are cheating on. He's cheating. Regardless of the reason and rationalizing going on, he is cheating.
I imagine you'd like him to leave the woman. If you think you can get over his lying to you for a long time, then offer that as a solution. If he won't, then you probably are best to cut ties with him. He has a lot to deal with in order to lead an authentic life. But you don't have to be his little secret. You deserve far more than that!
Let me know how this talk goes! good luck and hugs