If I were in a relationship, and the threeway talk came up, and she would only do one and not both, then it'd be a straight no to the first. Why should she get all the fun with a MMF threesome, and not allow me the joy of a FFM? Seems kind of unfair in the bedroom.

Then, if I suspected her of cheating, it'd be over. But if you two are not in an open relationship in the first place, and you're not trying to bring home more guys/girls into the equation, you should probably go ahead and break it off.