I think it says much about the sexual objectification of the female form. It is now so pervasive in our society that a number of women (as well as men) regard the naked female form or the sight of women engaged in suggestive poses/activities etc. as erotic. The internet has meant it is also far more accessible.
This quote from the Quit Porn guy:
"There's a real sense among women that it's bad, dirty, wrong and they're often unable to get beyond that."
Er, well yes - once you know the facts about porn, it is indeed bad, dirty and wrong.
As dittany said, though, the numbers of porn-using women are still likely to be miniscule when compared with the number of male users of porn.