You just tell your bf that you just don't feel the chemistry is right and that you aren't interested in his gf. Let him know you are supportive of his being with her, but you're just not feeling it, and are pursuing someone else. Tell him that from now on he should make sure to ask you first before alluding to his gf the possibility of any threesome happening between you all.
Then, just contact his gf and simply say something like, "Thank you for sharing your dream with me. I am flattered that you still think of me that way but, right now, I prefer to just be your friend and I am a little uncomfortable with any propositions from you for sex, so please don't ask me anymore. I also want you to know that I have already planned to bring a date to the bar on Friday night and that's where my focus will be - I hope you understand."
Last edited by nycindie; 09-18-2012 at 06:38 PM.