I put sexuality down to sexual attraction though, which I would say is not fluid, just has a lot of different grays along the scale. I think people change what they identify as simply because we're used to such rigid terminology, rather than them actually changing themselves.
Okay, I'll stop saying "someone" 'cuz I'm mostly just using myself as an example. I'm attracted to feminine guys, guys who crossdress, etc., but not to more masculine men (mostly - if I get to know someone that can affect how I feel). At the same time I'm generally attracted to women regardless of how feminine or masculine they are, and to people who are androgynous. I don't think 'bisexual' really captures my sexuality and for someone to call me that would feel a bit, I dunno... disrespectful? Can't think of the right word at the moment.