The only way you can, you specifically ask him if he's bisexual because you know that he's been out making out with other guys.
The LEAST you can do is ask him about it. Without bringing other people into it. You said its happened multiple times which means its likely to happen again. If you ask him and he denies it and says people are just making up things about him, then you decide who you trust more.
Further, you need to figure out how much it matters to you. Is this a dealbreaker for you or do you just want to know that he's being open and honest with you? What if he says to you that he is bi (assuming that he is), what do you intend to do with that information from there? I can't really tell you what to do but it seems to me that your boyfriend is cheating on you. And you already know it to be true. Perhaps you want some kind of closure or to understand why, but what if that doesn't come. You need to be prepared to either accept that your boyfriend might cheat on you or that you will need to take a walk.
It was written.
I wouldn't care so much about WHO he is kissing, but the simple fact he is cheating on me :I
ya, the cheating seems like the bigger issue. if he loves you but needs men too than that should have been something he told you before going to the gay bar, so that you could decide together what to do from there. not that i think you should ok his being with other guys, just that as of now he's lieing cheating and sneaking, plenty of people have open or unconventional relationships, but all parties need to be privy to this information, otherwise he's just cheating scum
It is very, very likely that he is also having gay sex with these men as well. I hope that you are always using condoms, every single time that you have sex with him as anal sex is a high risk activity for STI's. If I were you, I would just tell him that you know what he is doing. This is cheating and is unacceptable behaviour whether it is with a guy or a girl. He could be bisexual I suppose, but I think it is more likely that he is gay and he is just dating you for a cover story because he is not out of the closet yet. So he probably doesn't realize that people have seen him. Good luck and please update us and let us know how it turns out.