I believe it is a combination of several factors, and not some itch policy against adult content. If they had a policy against adult content, they would ban you, like gamejolt did.
Yet here you are, and are even allowed to link to your other sources of income.
But there are problems with payments. Like you found out with stripe. The cheap payment processors that are available in most countries have some issues with adult content. Which is outragoues, since they are only parasites to real credit card companies that would even handle payments for real life porn.
Itch could step up to handle finaces different for adult content, but this would cost a lot of money. If you look at actual real life adult content, we are talking about taking half the money and having a minimum payout of over 100 bucks. This will not work for small time indie game payouts.
I hope they will offer some forms of additional optional financial methods of payment services. They need to take away some of the cake they let Patreon have at the moment. Patreon does have policy issues with adult content. They will ban you for content that is ok on itch.
A finished game might be released on places like Nutaku, gog or even Steam. But there are not many places for unfinished games - with payments for the developer.