It might be because i don't understand sexual attraction in general, but i don't understand why people want to watch footage of two random people humping each other, let alone actually pay for it
@Sisu
Vicarious arousal and excitement.
Also... it's usually more interesting the more you know (or think you do) about the people involved. PWP (Porn without plot) is usually a "use and discard" type thing. The favourites you come back to, build a little story for you with characters you come to care about (or at least find interesting).
Or... consider how very popular erotic fanfiction and fanart is. These are characters the fans already know quite well, but probably have never seen resolution for all the romantic and sexual tension that built in the mainstream show/comic/etc. Also, maybe they ship a different couple than the real author wanted to pair up? That makes the sex scene a form of fantasy fulfillment!
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