Yeah, you agreed, and yeah, it's a problem that you didn't have clear boundaries... but I don't agree with what seems to be the majority position here: that that's the main problem. I think the main problem is that your boyfriend knew you didn't want this to happen, and did it anyway. Shaking your head "no" at him and acting upset, even if you said yes, should be enough. I can't imagine not taking that level of hint with my wife.

I'm very sympathetic with the following reasoning, as well:

and in my mind at the time, if me boyfriend wants to fuck my bestfriend then it's like they've already [done] it

It's true: what hurts most is the mental/emotional aspect, although the physical fulfillment of the mental betrayal makes it even worse.

I think a possible mitigating factor here is that you were comfortable with letting her watch, and even apparently -- to your boyfriend -- comfortable with her joining in. So the door's already opened, in your boyfriend's mind: hey, she's cool with sex stuff with this girl, so it's not bad if I go further. It's very easy for hormones and desire to take over from there, so that he might have been way more amped up and open than normal. He should have taken the hints, and it seems fucked up that he wanted to fuck her at all, but it was already a very sexual situation... so I guess what I'd be concerned with is, would he have wanted to even without all the foreplay?

I don't know if that's answerable. But, if it isn't, I guess you can decide whether the fact that he thought with his dick is forgivable or not, and if he's genuinely apologetic, and genuinely in love with you. I don't think you're overreacting, but I do think the bad decisions started at "everyone can sleep in the same bed" and didn't stop.