Since no one has taken this approach to answering OPs question, I'll say it.
It's biological gambling.
Someone mentioned fetishes predate porn, and essentially art and language. Well, I'd say it's pretty fundamental to life itself and has been for the last 4 billion years or so of it's existence, to experiment.
Look at the diversity of life, not just on the biological scale, but the chemical scale. Atoms and molecules, mixing with each other, finding combinations that work, and many more that don't. And by work, I mean go on to produce chemicals that aid in their own reproduction.
Almost all of the elements on Earth can be found and used in some way or another, in every large organisms like humans, reptiles and fish. Most of what the crust of the Earth is composed of is found in similar quantities, in beings like us. This, balance, between the chemical abundance of the world around us and the chemical abundance within us, is something life has been working on reaching for billions of years. Now there is an abundance of things like silicon, iron and oxygen, found on and within the earth that life is working with and as human beings take up a larger and larger proportion of the biomass on the surface, I think it only logical to expect some of it to 'experiment' with the rest.
It's kind of like the way people find use for industrial waste, like sawdust from a lumbermill, or the byproducts of refining crude oil, which go on to become things like plastic. We are doing, and will continue to do that, in biological ways, whether we mean to or not.
Sexual fetishes might seem more like a subject to approach from a psychological perspective, but at it's roots, psychology is just a product of life, and life a product of chemistry and chemistry is the marriage of material, not just on Earth, but throughout the cosmos.
Which makes me wonder, why have I not seen porn of giant human beings sexually engaging black holes? Humans that eat world and blow loads into dead stars?
