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, and possibly has a thing for your friend. 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, and the physical fulfillment of the mental betrayal is just the shit-icing on the pain-cake.

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. For example, I don't normally look at chicks who aren't my wife; I seriously couldn't care less. But if I'm already very aroused and we're fooling around and she suggests porn, I'm suddenly totally into it: she okay'd it, and I'm in a mental state to go with it.

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? Are you still first in his heart and his dick... so to speak?

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.