Perhaps more to the point, if you've selected not to see adult contents, why are you being shown 'adult' tags. It can't be that hard to maintain a blacklist of adult tags and simply remove them from the tag selection, then filter out any adult games that make it through in the normal tag search (i.e. furry or cats).
Viewing post in Delete all ADULT games
The system itself works quite well. And it does not involve tags at all, only the meta data. A game does not even need tags.
The issue is, that the system is ignored for certain tags. They implemented a whitelist so to speak. The fix is not to implement a blacklist on top to cancel that whitelist, but to remove that whitelist, as the configuration has to be done by the setting and not by the tags.
Oh, and half of the games tagged furry are adult games ;-) They get filtered out just fine with the setting.
As to why there are tags in the suggested list, that are common adult tags in the first place, including literal adult, well, we can speculate. Maybe they had too many complaints from people browsing that tags and not seeing any content. But instead of displaying a message that people might need to make an account and activate adult content (or changing the suggested tags per setting), they "fixed" it by having that whitelist. Or any other reason that seemed like a good idea at the time. (One should also not forget, that you can land on tag browsing by clicking tags on a game page, so tags need not even be in the suggested tag box.)
Also, there are games with some common adult tags, that are not adult games. As in, they have to be marked as adult. Maybe they even had at one time a mechanism that would add the marking based on tags, for developers that forgot to mark it in meta data. But strictly speaking, you can have the tag erotic, without the game being a M or AO rated game. It is only tags. Once could argue that the tag might be incorrectly applied. But even the suggested tag definition on itch for erotic only says: Usually, and most of the time.