My sexuality has not really changed much, until recently.
I've identified as gay since age 16, because for the most part, that's what made sense.
Over the past year, though, I find myself more attracted to females than I have been in the past, which is a bit confusing. And whereas some guys experiment with their male roommates when the mood hits, I have done so with female roommates, and I was totally into it. It's an odd reversal, to say the least. Also, I can really only get it going with straight porn, these days.
So, I get where you're coming from. I chalk it up to a change in tastes over time, like with other habits.
If you don't already, I might suggest listening to Dan Savage's podcast, the Savage LoveCast. This show has helped me make peace with the complexity of human sexuality by answering all the questions that I've had, and therefore, satisfied my confusion on the issue.
When it comes down to it, I think it's best to be open about one's sexuality with potential sexual/romantic partners on an as-needed basis, maybe explaining that your sexuality is fairly fluid, use that as a romantic filter of sorts, then work with whomever makes it past that filter.