From: In my chair
This is never one of those easy ones. On a very basic level, if your instincts are telling you that it is a bad idea, you need to listen. This is a situation where if you have to ask the question, you should not be doing it. But, also, you need to understand something about your best friend.
You know how your best friend seems okay with your relationship? He's actually not. He's in a very unhealthy situation, and in a lot of suffering because of it. I know - I've been there.
I used to dearly love somebody who would not date me. She'd date other people, and sometimes even form relationships with them. She's even tell me about it. What I came to realize was that we were dating each other emotionally, but not in any other way. And I could have happily chased her for the rest of my life, despite the pain it was causing me to not be able to be with the woman I loved. This lasted years - it was only after I finally realized that I needed to break it off that I was able to move on, and meet the woman who is now my fiancee.
(And yes, in that time, I was respectful of her relationships, and I told her I was fine with her not dating me - and I was lying through my teeth. I was lying to her, because her happiness was more important to me than my own, and I was lying to myself, because I could not face the truth until the pain became just too great to bear.)
Now, there was a bit more mutual emotional attachment in my case, but your situation is very similar. Your best friend is chasing you - so long as the merest possibility of having something with you, even if it is a junior partner in a polyamorous relationship, he will not be able to let go and move on. And he WILL suffer, telling you he's fine throughout.
I'm sorry to tell you this, I truly am, but you have to make a choice - you need to choose between your boyfriend and your best friend. And, if you choose your boyfriend, you need to make that choice crystal clear to your best friend, so that he can move on and get on with his life.
I wish you could keep both in this situation, but you can't. So long as you let the love triangle persist, you're hurting your best friend. If you try to keep both, it will last until your best friend's pain becomes too much for him to bear, and when you lose him, it will be very bitter.
Take it from somebody who knows.