Itch doesn't have a staff capable of reviewing every game before it's published.
Most games are published and indexed, trusting that the developers are honest.
Itch has a "bot" that, through a secret algorithm, can deindex a game and place it on a manual review list. This means that the game must wait to be reviewed by a human staff member, but this only affects some games, and many others are indexed immediately without any prior review.
When you find a game with adult content (especially pornographic) when you shouldn't, it's almost certain that the game wasn't properly tagged by the author (sometimes it's an honest mistake, other times the developers try to circumvent the system).
In those cases, as already mentioned, the best thing to do is use the report button at the bottom of the game to bring it to the staff's attention.
For example, the game "TAffy Tales," judging by the images, is clearly NSFW, but if you check its tags, you'll see that the author didn't include any adult content tags:
2D, Characters, Pixel Art, Singleplayer, Story Rich, uberpie
It's possible the author is trying to get their game indexed (despite selling it), hoping to slip under the staff's radar.