| A frequent problem when trying to figure out the right way to categorize (or pigeonhole) people. To take examples from my real life which will probably get my ass flamed:
I'm a male. Once upon a time I was dating a lesbian. Since I am not now nor have I ever been transgendered, we can safely assume I was male at the time. Now, this woman identified as lesbian but she was dating me. What gives?
Another woman I know identified as bisexual but only dated women. And another identified as bisexual but only dated men.
I identify as polyamorous but right now am dating only one person. I was once romantically involved with someone who considered herself monogamous and was dating three people, openly, at the same time.
Many people who identify as monogamous have undisclosed affairs with others, often who also identify as monogamous. Are these people monogamous, polyamorous, or what?
My general answer to this kind of question is I don't fucking care, but that's probably a bit glib and over-simplifying. At my undergraduate institution, a woman I know interviewed most of the lesbian population of the campus and discovered that over eighty percent of them were behaviorally bisexual. But they didn't consider themselves bi because a) they really did prefer women, or at least most of them did, and b) bisexual had a whole bunch of connotations for them that they didn't particularly like. Now, of course, there's the fact that some of those connotations are ones that bisexuals don't particularly like either. In any case, to these women, the identity of lesbian was an important one, even if their behavior was driven by factors other than the identity.
Sexual and romantic examples are simply the most dramatic. I identify as a bit of a linux advocate and yet most of the time I use windows.
Okay, this writeup is way too long now. | 
