well there was a thread on this awhile back took me awhile to find it but here http://emptyclosets.com/forum/support-advice/58692-sexuality-fluid.html personally i dont think its fluid, like TyRwar said its more of a spectrum, because i seriously cant imagine myself with a women sexually.
I feel it is for some people, your feelings can change, whether constantly or rarely. It's your feelings, it's not like you change them, they change themselves. That doesn't mean it's a choice. No one would choose to have confusing and frustrating feelings that you don't know what you are and who you like.
a year or so i go i put a poll up on the issue and alot of people had experienced fluidity. here are the polls: Female sexual fluidity Male sexual fluidity
Some people do drift or change in their sexual desires over a time. Example: I know a woman who was happily married with three children. The youngest child at the time of her epiphany was fourteen. Over a period of two tears she had divorced her husband, came out to everyone she knows including her very accepting children and married a woman she had known and worked with for twelve years. Some call this sexual fluidity. Other think it is an ability to move back and forth which I would consider bi. But that is my opinion.