tldr: Financial transactions for porn suffer VERY high cashback rates so traditional payment operators do NOT like them because they become expensive to them VERY quickly.
So, he does not trust third-parties with his banking details,
but uses a third-party to send emails with a complete detail of his account to an email-server (which might or might not be run by a third party).
The logic is lost on me.
If you go read the Legend Mode, you'll find mentions of Doomdice and you can look up quickly what happened in 1387.
Everything you find on artwork, is either related to the preferences of the dwarf who created the item, or to some of their known history.
tldr: Financial transactions for porn suffer VERY high cashback rates so traditional payment operators do NOT like them because they become expensive to them VERY quickly.