While skimming this hentai manga, I noticed that the male genitals are distinguishable while the female ones are not then I had the following revelation (keeping in mind Evolution):

One's genitals are less visually accessible for a female than for a male.
Thus, during sexual intercourse, it is easier for a male than it is for a female to take a peek at what is physically happening.
Therefore, a male is more easily aroused by graphical allusions to sexual intercourse than a female is.