I think either they're into it or they're not. My ex would have slapped me if I brought up a threesome one more time for the fact that she was insecure (she always thought I was looking at someone or wanting to hook up with someone) and she would act like thats gross. Okay, so she doesn't like the thought of being with another chick, I understand because I would never think of being with a guy. Fine. She doesn't have it. My current GF (you read about it in another thread; 3 some question) IS a little curious, not all out bi, but she thinks women are sexy and would dabble if the situation arouse. She has kissed other chicks and she likes it when I do, she said it turns her on to see someone want what she has.
So see my point is..... Either they have it or they don't....
If you care about her, stop bringing it up, it WILL cause problems (it did with my ex)