When i was younger all i knew was what i saw in movies and lets just say most of them didn't paint it a good rep. Yes there is the "Pretty Woman" and such, but most movies are more in the way of "The Equalizer" or such. Its really easy for someone who never been in the actual business to believe all the negative he is fed from medias both fiction and real life (news paper, tv news etc).
I think the whole "not paying for porn" comes from 2 things. The first is the obvious that porn been on the internet for as long as it existed and always was easily accessible. Companies themselves would upload clips of movies (wich at the time were only available on VHS and DVDs) and a lot of my generation seem to have grown up with this. I still remember waiting 15 minute for a couple megabites clip of 45 sec

It was done to create buzz and publicity, but it kinda conditoned an entire generation that if they wanted to see porn for a quick fap, they could easily. As internet developed itself more and more, torrenting appeared, then tube site made it even easier, and still to this day many companies upload clips of 5m of their paid content. Sure you don't have the full scene, but for some peoples its enough for their "needs".
The other is that lots of other type of content being available for free renforce that idea. For instance Youtube, it runs on ads but can be view totally free. So im sure peoples expect something similar out of porn but the difference is regular product ain't advertising on porn, its just "other porn" (like brazzers publicity, or cam sites). Its nowhere near as lucrative but the false sense of "there is ads, so its fine" is still felt.
But if i learn anything is that for all peoples not paying, there is often enough equivalent that will. For exemple many creators i follow on YT have a patreon and mention their backers in the vids, so these peoples pay willingly even if they wouldn't need to cause they like the content. Same for porn, how many guys throw tons of money at OF creators as tips or whatnot for litterally nothing (if only attention), and this create both a balancing (vs the non-payers) but also an unbalance because you end up with pricing that are way above what they should be for the content. The creator expect to make good money on the "whales" but then you have others who will pirate because its just way too expensive. Sure some would had pirate anyway no matter what don't get me wrong, but a good chunk may had bough it if it was 60% less expensive.
Anyway, i don't think the online stuff contribute to the "demonization" of this business like you imply in the last line of your post, but thats just my opinion. I think personally it only go as simple as the old days when sexuality was seen as that "secret thing", that you had to be married and so on. Then times changed and yes peoples fuck for fun all the time as ONS or such, but there is 2 other factors.
On the male side there is the ego. I never be secretive on the fact i see escorts, and i been told a few times by other guys that there is "no way they would pay for pussy". It would hurt their ego, AND they still believe the false stigma that escorts carry diseases (probably because they got one themselves from a ONS of a girl that sleep around). But what they don't know/understand is that they often pay themselves just differently (dates ain't cheap...lol) and that escorts values their work and get tested often and on the contrary are super clean.
And on female side, some consider giving their body for money "shameful" yet go around and sleep with tons of guys. Sure they choose the guy, but its still sex. Like whats so shameful of making money out of it?
Anyway i could go on and on and i know im maybe not completely precise in my text especially in english, but i hope my general idea is understood.