I have a theory. I could wander around the internet trying to find evidence that backs me up, but there's a lot of porn in the way [side note for later discussion-how disturbing is it that if you enter the words teen and lesbian you get almost nothing but porn? How bad is that for questioning youth?] but right, this is clearly just an opinion.
Teen magazines are often scolded for encouraging young females to degrade themselves to men. However, this may have been true in the past, and in some articles, but it seems the majority of the contents of teen magazines are the opposite. For starters, there's the pictures. While often cited for damaging girls body images with unrealistic airbrushed girls who are almost all white and such, a valid point, there is something people miss. When I was 19 I got free subscriptions to two magazines-Stuff and YM. Proportionately speaking, I noticed YM had way more pictures of attractive girls, often in revealing clothing. While this clearly has negative effects on young girls ideas of themselves it's also doing something else. There will be one or two shots of cute boys, or boys in pictures with girls where the girl is usually the focus. Page after page off attractive females are being offered up. Any girl even a little attracted to girls is bound to notice one she likes. Plus, you know, if your pervy, go buy YM, and if you want humor go by Stuff.
Then there's the content. For every article on "Does he like me?" It seems there are several on why you need to stick by your [female] friends over a guy, implications you can be closer to girls than guys, and horror stories of boys pressuring girls into sex, raping girls, convincing them to try drugs etc. I'm not saying this an unrealistic representation of reality. As for the fashion content, that teaches girls to notice other girls physically more. It isn't really about attracting guys, because the average straight male does not care how "in" a girl's clothing is as long as she looks good in it.
It seems pretty clear magazines are turning young girls into lesbians!
Okay, no. Nothing "turns" someone gay. But I honestly do believe that these magazines are encouraging, probably totally unintentionally, a questioning, a background idea that girls are more attractive and better to be close to than guys. I'm not saying there are girls who would be straight if they hadn't poured over seventeen when they were young. But perhaps part of the seeming upswing in teen bisexuals stems from the fact that girls are coming in touch with attraction to girls through these magazines. Also, lesbians are hawt.