I don't think it's proven that men are more visually stimulated by sexual images than women at all.
Interesting study done in 2006 says otherwise [Anokhin AP, Golosheykin S, Sirevaag E, et. al. ?Rapid discrimination of visual scene content in the human brain? Brain Research, 2006 May 16]
The study, carried out by researchers at Washington University School of Medicine in St. Louis measured brainwave activity of 264 women as they viewed a series of 55 color slides that contained various scenes from water skiers to snarling dogs to partially-clad couples in sensual poses. The researchers were interested in the speed, strength, and location of brainwave activity of the subjects as they viewed erotic versus non-erotic images.
As they hypothesized, the brainwave activity of participants was markedly different when viewing erotic images versus non-erotic images. But a finding they didn?t expect was that female participant?s response was similar to men. In a prepared statement, lead author Andrey P. Anokhin explained:
"Usually men subjectively rate erotic material much higher than women," he says. "So based on those data we would expect lower responses in women, but that was not the case. Women have responses as strong as those seen in men."
The authors propose that previous findings from other studies which found men to have a stronger response to erotic images than women may have as much to do with research methods, as an actual response by men or women.
So basically previous findings showing men are 'more' visually stimulated by sexual images than women are being doubted because of the type of sexual images show. Pornography is almost entirely geared towards men. Makes sense that it would appeal more to its target market. Doesn't mean sexual images per se are less arousing to women.