Like with many forms of entertainment, it's a vicarious experience.
"Monkey sees, monkey do" is one of the basis for learning among mammals and primates. Thus, there is a case to make that we are likely programmed to enjoy watching people doing things we would like to do. Porn is likely a
supernormal stimulus, but it's not qualitatively different from say TV, cinema and even theater.
The existence of so-called mirror neurons support this idea IMHO :
en.wikipedia.org
That being said, you're right : porn is cucked. I just want to point out that there are reasons for its appeal, and I think it would be naive to deny how difficult it can be to resist it.