My bf's best friend

So I just need to vent a bit. I've (23) been dating my bf (22) for 4 months now. We're pretty good about talking about insecurities and everything. Overall, I'd say communication is pretty healthy. We both have some insecurities about infidelity. We are open as much as we need to be about our sexual pasts.My bf's best friend is also gay and is currently engaged. A couple months ago we hosted a holiday party and things went smoothly. Before the party I learned that my bf has been in a threesome before (which I'm fine with)...but it was with his best friend.According to my bf, his best friend "pumped him full of booze," and got a bit grabby and they ended up having group sex. My bf told me that he isn't attracted to his friend in that way. His description gave me a red flag-I'm feeling a little jealous, AND his best friend could have potentially overlooked consensual boundaries. Either way, my bf assured me and brushed it off.At a later incident, my bf's friend did it again and my bf went through with it. Granted both of these events occurred before I knew my bf, but it still makes me uncomfortable. I don't understand why my bf put himself in that situation a second time...I'm trying to just be in the moment and reflect on it as a past experience but every time my bf mentions his friend...I just feel so uncomfortable. His bf hasn't spared details about some of his friends's other sexual exploits...which just make me more uneasy. I don't wanna diss his friend, but I don't trust him. My bf has stated that he's also not in agreement with many of his friend's exploits.At the holiday party, I met his friend for the first time and he seems nice, although we didn't really interact.My policy is that I can't control someone...they will make their own choices. I can make decisions concerning myself based on those choices.Anyway, thank you for reading!