Oh poor babby. This isn't right.
What was he thinking?! Honestly, things like 'honey, I think it's about time we tried a threesome' should be talked about in advance and not just dropped on your lap in a hotel room in a place where you either have to get out and get a room for yourself or just go along with it regardless of your feelings. I do realize that you agreeing to go on a weekend-trip and getting rooms together to his mind amounted to consent, but he should have asked anyway.
The most important thing: you don't need to be involved with her to be involved with him. It is helpful if you know and like her, but even that is not an absolute necessity (especially not if you love drama
). Triads are an ideal many aspire to in the world of poly but hard to come by in real life.
It seems that having met you, his fantasy life just got a hold over his real life sensibilities and he envisioned this full-blown three-way honeymoon that took both you and his OSO by complete surprise. Being bi-curious or even bisexual doesn't make you automatically interested in any member of any sex, as I am sure you know (he might not, though). Many bi-curious women aren't really interested in having sex with another woman in real life, much less having a full-blown relationship with one. So telling him that you are interested in maybe exploring this side of yourself doesn't make it okay for him to start living out his own fantasies without prior negotiation with the two of you.
Put some distance between him and you. And when you feel ready (i.e. when the mental image isn't an acute source of emotional agony any more), sit down with him and make some boundaries, if you still feel like you can and you should.