Threesome problems

Basically my boyfriend's best friend wanted a threesome with me and him. This made sense as me and his friend have always had sexual tension. Never spoke about or acted on it but you know when it's just there? So anyway, I'm not going to lie - we'd been consuming drugs and alcohol. We wanted to start slow as it obviously would be a bit weird for my boyfriend. So I got with him in one room and then his friend in the other. I then went back into my boyfriends room and asked if I could bring him in but he couldn't get it up so wanted to postpone the whole thing. 
Now all I can think about is his friend. And he's acting like he can't remember any of it even though I know for a fact he can. 
I am so sexually frustrated right now and I feel bad for even thinking it but they were the ones who brought it up? 
I just want your thoughts. I have a real problem with desiring people that I can't have. I never act on it but having them thoughts are bad enough. My boyfriend is okay in bed but not great. Which is why I got so excited that we were going to try this. I'M JUST SO FRUSTRATED.