And if you want some fan service that fits these requirements and is still wholesome I give you Goldenboy.

Why am I not seeing One Piece in here? It sexualizes nami and Robin but they are both over 18. There aren’t really relationships at all in the show unless you count boa hancock but that’s pretty tame and not a relationship

Now try and find that but not somehow linked to high school.

Here’s some genre keywords for a start:

Adult Cast

Seinen/Josei

Chances are if those shows involve underage sexualization/unhealthy relationships, it doesn’t endorse them.

Edit: Also look for anime with kids as target audiences, they should have none of those things. On further introspection, that sums up to “look for shows that ain’t shounen or shoujo” lol.