Oh god I would probably never datea a woman. Women are crazy.
Maybe your mind just hasn't fully opened up to the idea that being with a woman doesn't mean giving up men. Maybe you would be more open to it if you and your fiance shared a woman. I know that the last time I was with a girl, I felt like...had my boyfriend not been in the room, ready to join in, when she started making advances on me, I would have been much less comfortable.