Some History
I have (big surprise) an all time fantasy of a FMF threesome that no girlfriend has ever fulfilled.
I've discussed this with my current girlfriend, and at first (like well over a year ago) she seemed open the possibility, then as time went on she became less and less enthused by the concept even though she's curious about being with another woman.
I found out through various talks with her that she has the somewhat common woman's fantasy of being with two men at the same time -- though she's not sure if she really wants to try it for real, or just keep it as a fantasy only.
She also has another fantasy about being watched having sex, and through prodding I deduced that she would likely prefer the watcher to be another guy... and from further prodding she said something to the effect of, "whatever happens... happens". Which I took to mean that if it turned into a mfm threesome... that was all good. (She refused to elaborate, but I also seemed to note that she had a concern that having a mfm threesome would make her feel obligated to have a fmf threesome in the future).
She made it clear that she could certainly live without having a mfm threesome. As for myself, I'm not certain. I feel that if she won't have one with me, I may desire to seek one out on my own outside the relationship without her.
I have the desire to fulfill any fantasy she has, and I have a willing participant (not that finding another guy to participate in any level of sexual involvement with your girlfriend is all that difficult).
The Question
If we go ahead an fulfill her fantasy... and say it involves some interaction with my buddy... what if she decides that she doesn't want to reciprocate the favor with a future fmf threesome (of similar interaction)?
Could it be grounds for ending the relationship?