Magic Magnum said
I know, but it's the sheer quantity of girls who fall for things such as Twilight or 50 shades of Grey. It's not a handful of people with one of 1000's of turn ons. It's a very large if not majority sized portion of the female sex.
Because Twilight was aimed at young girls and 50 Shades of Grey has a metric fuckton of fetishes in it. It's hard not to find one you don't like in there, honestly. (That doesn't mean you'll enjoy reading it though.)
Literally Twilight is two extremely beautiful boys who are badass and yet soft and caring and blah blah blah, fighting over you Bella. It shouldn't be surprising that young girls like that in the same way that many guys fantasize about having two women obsessively wanting to be with him. The difference is that women tend to be more emotional, men tend to be more physical, and that's because of biology.
Again, this is very, very normal. I want to stress that.