The answer to this is biological. Among all forms of life, including humans, male is a poor performer, as far as sexual performance goes. A male of any species is generally never ill treated by females except for a short while after mating, which could be a reaction to the poor performance of the male, perhaps much below expectations. This 'ill treatment', shall be that after a sexual encounter, 'the female eats up the male partner' as in the case of certain spider, or some other posture, all violent.
When it comes to man, knowing well that a great agony is certain to befall on all men if sexual encounters are to proceed naturally, our forefathers devised various methods to reduce the possibilities of sexual encounter. Thus, we have sexual union prohibited on many grounds, of age, caste, relation etc. Looking at pictures and deriving pleasure thereby is one such way to escape from 'performance'.
As females are free from this performance anxiety, they have no such need.