It sounds like your boyfriend might be gay. I’ve told this story a couple of times this week, but I’ll tell it again. I used to date a guy who showed no outward appearances of being gay, he didn’t talk “gay” he didn’t walk “gay” and he didn’t act unusual around males. He had told me that he had had several girlfriends in his past and that he almost married one of them, but it just didn’t work out. But guess what, turns out he was gay! We’re a little older, so back in the day when we were teens and 20 somethings, it was absolutely not cool to “come out of the closet.” Subsequently, there were lots of men who turned out to be gay that either had girlfriends or even got married. When you are completely terrified that you will be found out, as hard as it may seem to swing the other way, that is exactly what a lot of gay people do (until they can’t take it anymore, they’re found out, or the times change).
But if you know any gay guys that are out of the closet, you probably know that they make tremendous friends. Likely because they have more things in common with women than your average straight guy. So this would explain why he has a female best friend and you as a “girlfriend.” He might not ever tell you that he’s gay, either, and that will be a problem for you if you can’t find out for sure.
There’s also the possibility that your boyfriend was in some way “romantically involved” with this other girl, even though yours and his definitions of “involved” might be quite different. He might not have wanted a real relationship (like a dating situation with her) but he might very well enjoy all of the “benefits” that come with this type of relationship. They get to play house, and wear rings, and say lovey-dovey things to each other, but he has no committment to her. And he gets to have 2 women. Bet he loves this situation. I bet you don’t. I would not be able to put up with it. This woman is not his sister, so IMO, it’s not very kosher of him to be so close and lovey-dovey with her. It’s kind of creepy. Plus if you stay with him, you will never truly be his number one.