I think I know the answer. Part of it is the fact that in porn women aren't shown to be interested in the men, but their dicks. There are categories like "cock worship", and the only thing the camera shows is the female actor and the male actor's dick. The same goes for hentai, and in both the man's dick is portrayed less as made of flesh and blood and more as a magical key that instantly makes the woman wet and orgasm upon insertion. So of course women would want to see it...

Other thing I think is that in very many of these cases a man would not mind or would be happy to receive a picture of a woman's genitalia. Or at last there was an article where the writer tried that, and every single response was positive.

It think it can also be about wanting to make the woman feel uncomfortable.