Man I am really tired right now and not in the mood to explain to someone with underdeveloped taste or boring fetishes something very simple and basic. Aaaaanyway because this is important and simple here we go:
We seen millions of porn pictures and memorized them all. We are looking for something new and something more expressive. We immagine being any character in the scene at the same time as we like. The more extreme the scene, the easier and more intense we can feel what we want, immagine a picture of a bulging balloon getting squeezed hard. That's quite exciting and intense the moment where it's about to burst, kinda like a CLIMAX you know? It's really pretty simple.
Next point: why demons/monsters instead of boring as humans? Because we have seen millions of porn pics and we know what a human dick can do and feels like not just by looking at it, but by default even looking at a white canvas... a monster/demon/tentacle monster/beast has sooooo many more options, INFINITIVE options to be clear. It's like tools or robots. Getting fucked by a wrench like dick? I work in marketing, and we had to design ice (for eating). so to design an ice you don't design the taste on the image you promote it with. You design most of all the fucking shape. Now what is more exciting to lick. The default ice ball? Or a ball with bumpy outcroppings on it and a hole in the center and the different of material/constistency? You wanna feel go bananas on your skin, in your vagina, around your pee pee and all your other orificies.
Fucking noobs...
