A Canadian study of patients watching porn found that straight women tended to be more flexible in their sexual preferences. The straight women got aroused watching homosexual and heterosexual porn, whereas men and women who like women (either bi or lesbian) tended to be more rigid in their preferences. Men however tended to have a stronger arousal reaction overall to the porn than women.
The scientists theorize that women have to be more flexible in what turns them on in order to be able to lubricate during any kind of forceful penetration, and thereby protecting their vaginas from tears or infection. Or less plausibly, the women were just less exposed to porn and therefore stimulated by the simple novelty of it.