I would definitely argue that porn doesn't help matters much, but a lot of these guys think that women sleep around a lot, especially with "Chads" and will cheat on them anyway first chance they get. Of course there's the pure virgin waifu meme as well, but there's a lot of contributing factors. The idea that a woman has had sex with someone other than you makes her a whore or a slut is not an uncommon one amongst those types though. Then again, it might be a desperate attempt not to come off as a "simp" to your peer group.
I get it though, it's easier to come up with an excuse not to bother, or to reject her in the case of a woman throwing herself at you, than it is to risk her meeting someone she likes better and dumping you for him instead.
I don't know how dating apps have affected dynamics but I'd be interested in discussing that in a different thread.