A few things I have learned about sexuality since coming out as bisexual:
Sexuality is FAR more fluid than people give it credit for--it CAN change! Many teenagers (especially girls) are going through an extremely volatile phase of their sexual journey, and feel like labeling themselves as "bi" is easier and more acceptable than saying "I don't know how to define my sexuality." Our society tells us that it's not acceptable to embrace that period of questioning and transitioning between not being a sexual person and being a sexual one (going from a pre-pubescent child to an adult). Some of them are certainly looking for attention and want guys to notice them, but I think more teens are really just looking for a label to best cover up uncertainty. It doesn't make it okay, but that is what we will keep seeing until our society changes and tells us it's OKAY not to know what box we fit in--or to not fit into a box at all.

I also personally believe that FAR more people are bisexual than we think, especially women--but they are made to believe that having such feelings/thoughts is not acceptable, so they cling to one label or another instead of embracing the full range of their sexuality.

That said, I COMPLETELY understand your frustration! I came out to my parents, and I think one of the reasons they don't accept that I'm bisexual is that they don't see it as a legitimate sexuality, for exactly the reasons you've mentioned. I always hated it when my peers "decided to be bi," especially when I rarely, if ever, saw them embracing it fully! I have come to understand more about sexuality as a whole since coming to college, though, and while I don't condone that behavior, I do understand a little more of where it comes from.