But, but, what on earth possessed her?
I think whitesugar touched on the answer to this. Boys send the girls explicit videos of (supposedly) ad-hoc pornography all the time. They huddle round in groups, watching them. They think it's really cool. The boys big up the girls who (allegedly) act out porn for real with them, and the girls who (allegedly) do it seem cooler than the ones who don't. Boys put down the girls who don't.
I'm pretty old, but I understand this. It's only a harder-core version of what happened when I was at school in 1970. Back then, it took guts to opt out of the race to the -er, bottom, and I'm sure it takes even more guts now. Girls aren't just fighting peer group pressures, but an incredible wall of media pushing sexual precocity.
So insecure girl gets tons of boy attention. Whoa, she thinks, I'm cool! She pours two litres of cider and an e down her throat, immediately followed by Skinny Boy's cock.
And then she finds out they were lying :( She's far from cool.