Look, its possible. Man landed on the moon. I've seen people fly. With the aid of airplanes of course, but flight did happen. I just recently watched a movie about two white people in South Africa who managed to have a Black child. My point here is, again, anything is possible.
Now, is it recommended? No. It's nearly impossible for two people who used to date to be able to completely just be friends. I don't know your dude or his ex, but I imagine that they steer into inappropriate on occasion. Not saying that anybody's cheating, but that two people with such close ties is probably rife with opportunity for trouble. Hell, a man and a woman just being BFFs is difficult enough. But actual ex-lovers? Hell, I know that I wouldn't trust that for anything.
And I can also admit that its about me. I wouldn't believe that they'd be able to play it 100 percent cool and that feelings must still be there. They're exes that not only get along but STILL adore one another on some BFF steez? Nope. Not buying it. You have every right to be uncomfortable. . Now the tricky part is how do you bring that up without giving him an ultimatum.
I'm not sure you can. Just realize that bringing it up to him is going to result in him telling you that nothings going on (could be true) and that you need to trust him. And if it goes further towards you telling him that he needs to choose, don't be surprised if he chooses her.
Sorry, its pretty much a no win. You either accept and deal with it or be prepared to have to move on.
My best friend, with whom I share an apartment, is also an ex-boyfriend. It actually bugs us both the way that people always assume that we're secretly still in love with each other and will eventually end up admitting our feelings...we are friends and that is it, so yes, it is possible. Of course there's also a possibility that the OP has genuine reason to worry, but if he's never given you a real reason to worry then why should the poor guy be forced to choose between two people he cares about?
He's known her longer than you, possibly before he even met you. On the one hand, then, it would not be fair to him to sever all contact with her. On the other hand, though, you have valid concerns, especially if you witness any flirty behavior between them, and she was a woman that he was intimate at some point.
It's not entirely unheard of for exes to be good friends, though extremely rare, it does happen, and more often than GBFF makes it out to seem. Heck, I met a woman who was introduced to her now husband, by her exhusband. They been married over 20 years, the ex remarried, and she told me the four of them occasionally go out together.
It comes down to trust; if your BF done anything that proves he can't be trusted when he's around her. Regardless, you should, gently, in a non-confrontative, non-accusatory, non-demanding way, let him know you are uncomfortable with the level of closeness. Just don't be controlling yourself; you wouldn't like it if he told you who you could or could not have as friends.
When I met my boyfriend he had just broken up with a girl a month prior. They weren't really boyfriend/girlfriend but there was a lot of sex. He wanted more, she didn't, but they stayed friends. She's still his best friend today. I'll admit, the situation caused a slight hiccup between us. He admitted he still had feelings for her... and I said "Of course you do, iggit!" Just because there are feelings there doesn't mean you don't matter or that he's going to cheat. We worked through it. Any man worth his weight will be honest with how he feels and honest with you about everything else. Without that pillar, you have nothing.
I'm friends with his best friend and I trust her too. That helps a lot. The best thing you can do for yourself? Genuinely get to know the girl he says is his best friend. You'd do the same if it were a guy, I'd hope. Knowing her character will do a lot to assuage your (reasonable) fears.
Yep, having feelings is one thing, one that you can't always control. Acting on them is another thing, and one that you have 100% control over.
You can definitely be friends with your exes. BEST friends? As in, whenever something happens, good or bad, she's the first person he calls? When she needs advice, he's the first one she goes to? Yeah, I can see how that would be disconcerting. If they're actively flirting on top of that... No, definitely not a comfortable situation. However, unfortunately, it is also a no-win situation, as Panama said. Either you bring it up, he gets upset that you don't trust him, and if you brought it up in any way resembling an ultimatum chooses her, or you quietly stew in resentment until you feel you have to break up with him.