Itch's content filter could be better, that is a given. They only have yes/no for content that is not suitable for minors or the workplace.
That sums it up. Itch has no content evaluation. So you either browse for all content or only for the content that was not marked as unsuitable for minors or workplace.
The now hidden adult tags will circumvent the filter, in case you did not know. If you select tags like "adult", you will see such games, even if your account's settings are to not show them. They removed the adult tags from the suggestion box as a fix for the obvious issues this presents.
If you browse for the mature games that are not of lewd nature, you should know at least some of the usual tags they use. You typically won't find them under puzzle games. They have a reason for their mature nature and that usually involves some kind of horror or gore or violence tag. While there are lewd games among those too, it is rather uncommon. And you typically can recognise them and avoid them at a glance.
This goes for all tags and those tag discussions. Yeah, some people think that negative filtering would help them find games. But there are just so many games here. You need to positivly select your interests first to thin it out. And if in the tag combination of 2-4 tags most games are not what you want, then your tag selection might be lacking.
that doesn't solve the issue of a bunch of games that might use it for reasons that aren't centered around sexual gratification
The intent or artistical value is not considered in this. If there are pixel boobs, on Itch, it is 18+. If the developer did not mark it as sensitive, some complainer will complain.
The tick box a developer has in the metainfo shows this:
Has sensitive content — This project is not suited for minors or the workplace
Maybe this explains why you noticed some games that are not exactly 18+ still being marked as such. Sensitive is quite vague, also the workplace thingy.
I want to use Thea 2: The Shattering as an example. On Steam you can read this:
The developers describe the content like this:
The game contains mature storyline, including text descriptions of bloody sacrifice, sexual assault, violence against children and animals. A small number of artworks contain nudity.
On Itch it would be 18+, because of the nudity alone. Feel free to check what age rating it has on Steam in your country. It is displayed as 12+ the way I access it. That is 1 year less than the minimum age a user on Itch has to have according to tos.