This has nothing to do with individual games.
Try for yourself. This takes like 10 seconds. Just open an incognito window of your browser, go to itch, go to browse, select tag adult. Or Erotic. Or Eroge. Or type nsfw (you have to type it, because nsfw is not a suggested/featured tag. it is not in the list ;-)
This also happens, if you have adult content deactivated in your settings and go to browse and select adult tag.
While this might be intentional, I doubt it is. And if it is, I will call it bad design. It circumvents the specifically designed button you have to press to enable adult content. (Show content marked as adult in search & browse)
I reported this weeks ago to support, but maybe I was not understood, as you just demonstrated. you would not have attributed it to individual games otherwise.
It only works for ... non-adult tags. Including "sex".
Also, marked as adult should be a searchable tag. I shall make a thread about that.