If I had made a game that isn’t strictly for adults but still covered mature themes, then I would tag it as a game to be filtered out. I would expect that teenagers that think they can handle adult content and got the okay from their legal guardians would browse itch with that kind of content enabled. And those who don’t don’t.
There are several tags that trigger that warning besides NSFW. As far as I’m concerned the Adult tag would seem to be the closest to what you are looking/asking for. While things with that tag could include sexual content they don’t necessarily do. (The current definition reads: “For adults, often containing sexual or mature content.”)