Only option I've found is:
- Create two games on itch, a Demo and a Paid.
- Restrict what can be done or length in the demo version and set it as free
- Put the other one as paid and set a price
- Itch will delist and block sales on the paid one but allow downloads of the demo
- Set up a google drive and add the game to a downloads folder
- Set up a Patreon (or a basic webstore)
- Add a single Product, set it to your selling price and make the product a link to the drive file. Also add in a business email and say that if the user wants a permanent game key, to email.
- Advertise the product link on the Demo and Paid itch pages
- When people email, generate and send a key on itch
It's pretty convoluted, but it means you can distribute your game and gives customers the option between an immediate download or a permanent key. You meet the itch requirements by accepting payment for an adult game externally. There might be better ways of doing it, or automating it, but not figured them out yet.