So, this is a slow building situation...
My boyfriend of over 2 years has been best friends with this guy he's known since preschool and they're very close, always have been.
I've been friends with both of them for almost 5 years now (I did not actually start dating my current boyfriend until after a few years of being friends) and the man who I'm with now always showed interest in me and actively pursued me after I went through a break up with a previous boyfriend. His best friend never showed any interest in me or gave me much particular attention, even as a friend. I always kind of felt like his "best friend's girlfriend" and not much more. Even during the past 2 years he's treated me similarly, courteous and polite, but never over friendly or flirty.
But the past 6 months I feel like I've noticed subtle changes in his behavior and I'm not sure how to interpret it.
He makes small comments all the time towards me that no one else seems to catch. He'll compliment my clothes, how I decide to style my hair, or my taste in books/websites/movies. He'll agree with almost everything I say now or will take the time to have lengthy conversations with me about deeper topics. He never did ANY of these things prior to this past 6 months. He'd barely pay me or my opinions any more mind than a friendly stranger.
One time, when my boyfriend went out to breakfast with his parents, his best friend invited me out to lunch while we both waited for him to get back (I was living with my boyfriend at the time and his friend had stayed with us on the couch the night before). The whole time he was extremely nice to me, talkative, and offered to pay for everything. I felt like I had been on a date by the time we got back and didn't realize it until it was over.
He also texts me pretty consistently now just to talk about things (we're both veracious readers and my boyfriend, not so much, so we mostly talk about books) or snapchats me selfies of himself (some with no shirt on!).
I've been ignoring these signs for the most part, signing them off as me being paranoid.
But now I have a new problem. My boyfriend and I have recently been discussing trying out a threesome (I'm bi and expressed the interest myself). We agreed a threesome with another female would be fun to try first, but then I said I'd also be interested in doing one with 2 males if that went well. We agreed we'd each get to pick the person of our own respective gender to join so that we'd be comfortable with it.
I am almost 99% sure this past weekend he told his best friend that he wants him to be the one to join us as the 3rd male party. His best friend made a comment to me after we'd been hanging out all night and drinking that went something along the lines of, "By the way, thanks for not trying to **** me tonight." But he said it so low and under his breath I wasn't sure what to think so I didn't say any response.
I'm freaking out because I haven't told me boyfriend anything about his best friend's behavior but I DON'T think it's a good idea for him to be the 3rd person because of the weird tension I've been feeling. I don't want all that tension to come crashing down in front of him as he watches us interact intimately. I think it would turn out horribly.
So my question is this: Should I tell my boyfriend exactly why I don't think his closest friend should be the choice for our threesome or try to distract from the topic and convince him to pick someone else? I feel like he wouldn't want to pick any other guy because it's his best friend, and the guy friend he trusts most (which is probably why he trusts him to be in a threesome with us and it not be weird)?? But I also don't want to create drama around something either! I mean, regardless of the comments he makes or how nice he all of a sudden is to me, he technically hasn't tried anything funny. He's never touched me or tried to kiss me or said anything that's directly hitting on me.