I was in a similar situation. I had been with a woman once, but I was still quite curious about those instincts/desires as I dated men. I ended up in a relationship with a wonderful guy that lasted 5 years. We thought we were going to marry each other. But eventually, as you suggested you fear, the thought that I could go through my life without really exploring that other side of my sexuality caused me so much grief that I had to end it. It felt unfair to him to continue mulling over this while I was apparently "committed to him". Breaking things off with him was the hardest thing I've ever had to do. I loved him so very deeply, and I still do to this day, but I feared I would cause him and us harm in the long run by denying my feelings (or the possibility of those feelings) for women. Things between us 6 months later are still strained, but I'm hoping time will heal things.

Now I'm in a wonderful relationship with a very lovely lady. When I look back on that time, I realize that I made 100% the right decision. My feelings for women weren't just curiosity, nor was the desire sated by this relationship. I feel now that I've opened up a whole new world that I really enjoy... one that I never would have known much about had I not taken that leap.

Who knows if I'll end up with a man or a woman. I don't particularly care, as long we love and respect eachother. But that's just me.