I thought the adult rating was only for nudity and sex?
In my personal opinion, the gore isn't THAT bad...
Besides, doesn't marking a game as Adult hide said game from general audiences?
Itch has no actual age rating. Their effective rating is 13+ and 18+. You need to be 13 to use itch according to their tos, and they have a section that you can activate, that is in all intents and purposes for adults. But is vaguely named non safe for work. Or not suitable for minors.
Age ratings for games and movies are different around the world, and how they are enforced. Itch is US based, and the US ratings for video games are E (0), E10, T (13), M (17), A (18), with several additions. Also different around the world is, what would require a higher rating. Violence in general will give higher ratings in Europe and Sexual content will give higher ratings in US and vice versa.
But your game is not officially rated anyways. Maybe it will be for your Steam presence.
In practial terms, unless it is porn or graphic ultra violence, it will be a rating of less than 18, and itch does not care if it is 13 or 17.
From your comments I gather, you would rate it 17.
Steam might give you this warning message, if they also deem it 17.
THIS GAME MAY CONTAIN CONTENT NOT APPROPRIATE FOR ALL AGES,
OR MAY NOT BE APPROPRIATE FOR VIEWING AT WORK.
Nudity is not considered adult only. It might be on itch, since they do not have any 17 rating or other mature content labels.
They vaguely tell you this: "If you have a project page that has strong adult themes, then it must be marked."
Whatever that means. What is an adult theme? What is an adult theme but not a strong adult theme? Is the graphic depictions of those themes relevant? And so on. And what is considered adult or strong adult would also vary with your location, but the developers come from all around the world with different cultural standards. You can show nudity in day time tv in Europe.
In practical terms, sexual stuff has to be marked. Nudity might need to be marked because of their equally vague not safe for work wording. Gore and violence might have to be marked, but looking at all those horror games, it might not be enforced for all but the most extreme games.
A notable non sexual example would be Fear & Hunger. It was released on itch and steam. But it never got an official esrb rating. Yet it is restricted on Steam and on Itch, maybe by the creator's own rating, maybe by people categorizing it.
Pragmatically, if youtube restricts streaming of the game, it probably has to be marked. And if not, then probably not.
The and in English is not a logical AND operator that has imaginary brackets around the operands. The concepts exist separatly and if you cite them together with an "and", you do connect them, but not to make them a new term, but to apply the thing said before to both.
A new term would have been sexualized nudity or nudity with sex. As the esrb sees it, they have their own category and you can get an esrb rating separatly for sex, and for nudity, and in varying degrees. https://www.esrb.org/ratings-guide/ They do include partial nudity in "sexual content" though, so you will get that one for free ;-)