babe, fantasies are fantasies. I have known that I was bi-sexual from the age of 15. How you conduct yourself is not about your sexual orientation, it is about your value system. I have been with my partner for 5 years, we do sometimes end up appreciating the same pretty girl walking by!!, but thats all. We both know where we stand and I would no sooner be unfaithful to him with a man or woman.
Try and not get hung up on it........you are not doing anthing wrong and i do not necessarily think that you are bisexual.......more just a harmless and enjoyable fantasy. The only time you may have to reassess is if you met a woman and fell in love with her, then your emotions may become very conflicted.
Stupid question maybe, but have you talked to your husband about how you are feeling? It may help......
Good luck and stop being so hard on yourself!!......
I have fantasies about vampires, figure that one!!!