Note: I'm not a lawyer, just some guy that know a lot about the subject and is sharing his knowledge. Therefore, in no case this can be take as being a legal advice.
I could be mistaken as I'm not terribly familiar with music copyrighting, but wouldn't copyright free music relinquish all rights the holder has to the music? CC0, or Public Domain images (Old images, or even Pixabay/Unsplash), are copyright-free images that relinquish all possible copyrights and/or licenses for commercial and personal use - regardless of how the model and photographer feel about the project. Given the name, and open use of a track for free without pay, I don't think an artist would have any legal standing if someone used their song/track/effect in an adult game, no matter how much they dislike it.
In fact, "copyright free" is a language abuse. By example, Creative Common licenses aren't copyright free but more licenses "with more rights granted to the user".
In regard of Bern Convention over creation properties, and in regard of almost all the Laws over the world, a creator don't just own one right but many. Among them the rights of reproduction, use, distribution, modification and the right to sell the creation. A license said "copyright free" just mean that the said creator relinquished to some of those rights, but not necessarily all of them.
By example, the GNU Public License grant you the rights to reproduce, use, distribute and modify, but not the right to sell, that stay the property of the creator.
But, even while he relinquish to some, or even all, of his rights, the creator have perfectly the right to decide under which condition he will do it. You'll then have the right to this or that, as long as you respect those conditions. Fail to respect them, and then you lost the rights that were granted to you by the creator.
By example, the rights owner over the image of the Eiffel Tower have decided that the day time image is part of the domain public, and so anyone can photography or film it, individuals as well as professionals, without any fees nor restrictions. But the said image at night is only "copyright free" for personal use. If, as professional, you want to take a photo, or make a movie, with the Eiffel Tower in the background, you'll have to have the authorization and pay fees for that. And it's not due to the lighting, that is also under copyright since it's also a creation, because this apply whatever the moment (at night) you take the photo or film, while the lights are on only 5 minutes every hours.
Finally, have to be added that, at least in most Western countries (I don't remember for the Bern Convention) the property can't be conceded. Whatever how much you'll pay, you can buy all the rights over a creation, but it will stay the property of its creator. This imply that even if the license don't explicitly state that you can't use this music in an adult game, the creator can still decide afterwards to revoke your right to use it, this even if the music is said "copyright free". This at least if he's doing it before what will be seen, legally speaking, as the first public release ; and since the Patreon business model is to release update by update, there's risk that Justice see those updates as alpha or beta versions, and so state that the said "first public release" happen only when the game reach its final state.
It's why, by example, Banksy was able to partly destroy a creation sold by someone who isn't him, to someone who also isn't him. The buyer could have asked for compensation if he thought that it lowered the value of the creation, but at no time he could have sued Banksy for destruction of some private property ; because it's the property of Banksy before being the one of the buyer.