There's no set law that says you can't change your mind. If you're not sure I think you have to go w/ your gut & find out, but you said you're in a serious relationship w/ a man. IMHO you owe him a serious talk about how you feel instead of going behind his back. It would make it a but easier for both of you. Then find out what you're drawn to the most.
Not sure if this helps, but I wish you luck