The reason Paypal doesn't allow adult content creators is because payments of that nature are notoriously prone to chargebacks, cancellations, and other things that cost them money, time to sort out, and resources.
And they are correct, that does happen to an absolutely ridiculous degree. I'm married to a popular porn artist, this happens to him sometimes and he's not even that huge of a player in the industry, compared to sites or services.
They're cool with Patreon because Patreon handles all that if it comes up, and it's increasingly difficult to scam on Patreon, so Paypal doesn't have all that extra trouble with them. It's not a moral thing, it's just unfeasible from a business standpoint.