Is there a way to search for games on Patreon? I do not know of any. I think on Subscribestar you can search for similar accounts.
And Steam does not count in my book, because your product has to be near finished. They have quality standards on top of content restrictions.
You cannot buy development version games anywhere that I know of, except on itch.
On Patreon and Subscribestar you only get temporary access to a snapshot. You do not "buy" the game there, only whatever bonus the developer might give you. Same for other "donation" sites that enable devs to get money somehow.
The reason why people "sell" games on patreon/subscribestar is as you said. There is a market for that service. And that service is buying indie adult games.
(Oh, and the example above with the 50% or so was for real life porn. That is what those cam girls pay.)