my 2 cents...
your sexuality is up to you. identity isn't "correctly" labeling how you feel, it's presenting yourself to the rest of the world. that is, it's not "am i gay" it's "do i want to tell everyone i'm gay".
for example, i have a girl friend who identified for a long time as "bisexual", but she preferred women so when she dated girls she identified as "lesbian". she started dating this woman who came to her one day and said that she wanted to transition into a man.
here's where it gets interesting. my friend's "gf" was then faced with a decision to identify as a "trans man" or, once 'his' transition was complete, to identify as a "straight male". identifying as "trans" would mean that, even once he could pass as a man he would still be letting people know that he had transitioned. if 'he' ID's as "straight" then most people would never know he wasn't born that way.
furthermore, during 'his' transition (as is largely typical for transitioning individuals) he reached a point where he was interested in sexual encounters with men. men who were born men. when 'he' had never in his life been interested in men at all. something about the level of hormones mixed with having the body you've wanted your whole life induces a desire to try things you've never wanted before. does that make 'him' a gay man? he still has a girlfriend. or a 'straight' man with 'gay' tendencies? he wouldn't be telling many people that, at least not outwardly.
his identification directly affected hers. she had been, "bi" but was identifying as "gay" because she was dating women almost exclusively. now, her girlfriend is identifying as a "man" (trans, or straight) so what does she tell people? "i'm a lesbian, this my boyfriend"? and if she does that, is she obligated to explain that 'he' used to be a woman? what if he ID's as a straight man, where does she fit her lesbianism into a world that now views them as a straight couple? her actual preference never changed.
you can't point at someone and identify them. well, i guess you could but it's not really up to anyone but the person with the identity. if you have a fling and you don't want it to be a big deal, then it's a fling. (i really liked the shower-head/muppet analogy).
but if you have a fling and it's something you need again...well, your identity is largely going to affect your ability to seek that pleasure out again. as a lesbian, i'm not about to get seriously involved with someone who absolutely will not admit she likes me back -- no matter how many times she gets in my bed.
maybe that got way more involved than was intended. i could write a whole dissertation on this... i guess my point was that there is really no solid answer. "sexuality is fluid" is one of the number one mantras in the gay community.