Here is a picture of Anna. How tantilising, if The Green Man was interested making some real bucks he'd now be saying something like "Pay me $10 a month and you can see Anna and lots more like her with much more clarity."

A number of now extremely wealthy entrepreneurs have discovered that there are an aweful lot of people, presumably male, who are prepared to part with the $10 a month. Porn has established itself as one of the niche markets for which the internet is ideally suited. Pornography is one of the few things that that people, in particular men, are prepared to pay big bikkies for on the net.

Robert Deaner a neurobiologist at Duke University in Durham, North Carolina has found that it is not only human males that are prepared to pay for porn. He has found that male monkeys will 'pay' in fruit juice to look at a picture of a socially dominant monkey or a female's hindquarters. The monkeys could choose to take a smaller container in juice in exchange for a 1/2 second glance at a picture of the backside of a female member of their troop. Dr Bob says

Virtually all male monkeys will give up juice to see female hindquarters. They really value the images."

He goes on to say

This is not monkey pornography. It's more to do with assessing sexual receptiveness, which in the wild also involves females' behaviour and smell.

Clearly the human male audience of human porn gets some considerable pleasure from viewing it which is why they pay. Do the monkeys gain pleasure from the images? Dr Bob doesn't think so, he thinks they are paying to be kept informed of the sexual readiness of the females rather than enjoying the images for themselves.

It seems a bit of a fine destinction. Placed in the context of the humans greater ability to abstract concepts this may be essentially the same thing.
Bearing in mind that the definition of pornography is "Sexually explicit pictures, writing, or other material whose primary purpose is to cause sexual arousal"

Read more in Nature.

By way of a footnote the picture of Anna that I blurred out was from a post on male sexuality that I wrote some time ago. I was going to make you search for it (your equivalent of the sacrifice of fruit juice) but I'm not conducting any research so here is the link.