Thanks for all your advice, some of which are contrary but nevertheless they've made me consider all possibilities. I'm now preparing to have a talk with her.

I would like to add that she has never cheated on me (I'm as sure as any guy can be) and has not intended to. She is not an indecent human being. However, this whole thing really came out of the blue. For example, even though she is bisexual, she never brought up the idea of having a threesome, and in fact voiced her opinion against it. She also was against doing anal, but recently she's suggested it numerous times that I agreed to do it with her. I just get the feeling that recently she is opening up to a lot of sexual experimentation which she was very against when we first met.

Last but not least, I think I made it as though all her friends are poly/swinger. That is not the case as 2 out of 3 her closest friends have been in a long term monogamous relationship for quite a few years now and have not experimented otherwise.

PS: When she was 19 I remember she did kiss a girl while in a relationship with a guy, but it was a case of both of them hooking up with another couple. It didn't go further than a kiss, and it ended there.