Well, I think the social pressures on guys and girls are very different.
Teenage boys are expected by society to want sex all the time, and talk and brag about it with their mates. And it's taken for granted that almost all guys masturbate, and that they might want to look at porn while they do.
Teenage girls are expected to be less interested in sex - if you're "too" interested, you might be portrayed as "easy" or a "slut". So it wouldn't be seen as normal for a teenage girl to talk about feeling horny all the time or brag with her mates about being sexual. The fact that girls masturbate is still pretty taboo, let alone the idea of wanting to look at porn while you do it.
Of course, these are vicious stereotypes, and have no relation to people's actual feelings or behaviour. But people are often pressured to conform to the stereotypes or get punished by being seen as "weird" ("slutty" or not a "real man" or whatever). So it's hard for people to talk honestly about what they actually feel and do.
Another factor is that the bulk of easily-available porn is aimed at a straight male audience, with a large minority aimed at a gay male audience - porn made by or for women (of whatever orientation) is much rarer. So even if girls are just as interested in porn as boys, they may have a harder time finding porn that they like (and the same goes for boys who don't like the sort of porn that boys are stereotypically supposed to like).
[This message has been edited by logic_grrl (edited 05-19-2002).]