A good number of people, if not most, have at least some bi-curious tendancies, especially girls (or at least girls tend to be more open to the idea). I would consider myself bi-curious but still am more straight and have never had any experiences with other girls, so can't classify myself as being bi. I understand why so many people would though. Younger girls, from their early teens to their 20s even, have crazy hormones going on and some may be bi for awhile and may shift out of it after the hormones die down, or it could be a lasting thing throughout life. I just think it's more common for girls my age to consider themselves bi because it's all about experimenting at this age.
I'll give em' benefiet of the doubt tho, and trust they are putting that down cuz they are bi and not just doin it for looks.
Because 13 year old girls want attention, and it's "in" to be bi, because "aMaGaWd AlL tHa BoIs ThInK iTs SoOoO hAwT!11oneone!11shiftkey!!1" And nobody's sure of their sexual preference when they're 13.
There's a difference between women who get drunk and kiss girls, and women who are actually attracted to women sexually and romantically. Both groups often define themselves as bisexual. Not saying that you've actually had to have sex with a woman to be bisexual, but if you're only "bi" when you're drunk or high, I don't believe you're really bisexual-you just have lower inhibitions due to intoxication.
To be honest it is because girls are very attractive, even if your aren't bi its hard to not admit that the female body is extremely sexy...
