Thanks all for the replies.
I should probably clarify something. We are engaged to be married but that doesn't mean we haven't been together long. We have been together for over three years and engaged for about one and a half years. The only reason we are not married is that we don't make much money and we want to have a nice wedding and not just go down to the courthouse and get hitched. I just wanted to mention this so that you didn't think we are a couple who barely know each other yet.
I don't think it would be possible to be emotionally closer to someone than I am with C. That's why I am pretty sure she isn't just telling me what she thinks I want to hear. The concept of letting her have sex with another person is not a new one, it's just that before the fantasy was exclusively me watching her with another woman which she is seriously into. I don't really have any reservations about her playing with a girl, which seems to be pretty common among us guys. I guess it's almost a double standard to be all rah-rah about her being with a girl and scared about her being with a guy, but it's probably because I don't see other girls as potential rivals for her love and of course there is the whole feeling of possessiveness (it's just a gut feeling, I know I don't own her). That's the thing though, those are just knee-jerk feelings. I know intellectually that she is probably capable of having sex with anyone else and will always come back to me.
Anyway, don't worry guys we are not going to rush out this weekend and try to get it on with someone. I know she and I will be doing much more talking about it before anything gets even close to happening. The talking about it is half the fun, it's exciting to talk about something new like this with her.
B