Meredith Chivers, a psychology professor and sexual research writer, organized a study in which male and female subjects viewed pornography consisting of various men/women combinations, while their arousal levels were measured by instruments, as well as through a keypad which they used to rate what they liked.

This piece in the NY Times Magazine offers insight into the female sexuality, and how it is a bit more complicated than many believe, often showing conflict or complete disparity between what women say arouses them and what objectively arouses them:

No matter what their self-proclaimed sexual orientation, [the women] showed, on the whole, strong and swift genital arousal when the screen offered men with men, women with women and women with men. They responded objectively much more to the exercising woman than to the strolling man, and their blood flow rose quickly — and markedly, though to a lesser degree than during all the human scenes except the footage of the ambling, strapping man — as they watched the apes. And with the women, especially the straight women, mind and genitals seemed scarcely to belong to the same person.

via What Do Women Want? - Discovering What Ignites Female Desire - NYTimes.com.