Pornography is, what economists call, an 'inferior good'. The superior good in this case is sex with (an) attractive partner(s). The average male cannot afford a large number of attractive super-models to be at his beck-and-call, and some choose to consume an inferior good - a substitute - pornography.
In fact the high availability of pornography which the Internet has brought has decreased the value of other substitutive goods - like sex with a partner.
This is why many feminists and other advocates of women's rights are so opposed to the easy availability of pornography. Fewer men will bother to enter marriage, or relationships, and fewer will be as tolerant in those relationships because of the high availability of the substitute.
Pornography may be an inferior good compared to real sex, but it is a lot cheaper than the divorce which about half of today's marriages end in.
We often hear feminists describing the sense of 'worthlessness' they feel when men around them look at pornography. This feeling is real. Women have controlled access to their sexuality for millennia, just like a product, and in turn they have controlled men around them with it. A substitutive good (even an inferior one), lowers or 'cheapens' the value of their own.
We saw a similar feminist backlash in the sexual revolution in the 1960s.
One of the ironies of pornography is that it seems illogical. Most things we do can be explained in terms of Selfish Gene Theory and helps our reproductivity in some direct or indirect way. Viewing of pornography doesn't seem to.
The answer of course is that humans didn't evolve in an environment where pornography was available, and hence there was no need for a gene which discriminated between it and 'the real thing'. Maybe in another 500,000 years there will, but with the amount of genetic tampering, it is hard to predict.
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