Even if it is the sub plot, just no hs age stuff. I love most genres, but action, fantasy, physiological is typically my jam as long as there is some sort of love story involved. Also feel free to recommend me something on my planning list if you find something that stands out.
Planetes
Soukou Kihei Votoms
Arakawa Under the Bridge
Basilisk Kouga Ninpouchou
Servant x Service
Arte
Kemonozume
Megazone 23
Rec
I want to warn that Kemonozume, and Megazone 23 are NSFW as there's some nudity. Basilisk has over the top violence too.
Spice and Wolf MERCHANT MEETS THE WISE WOLF
This anime is not entirely about Romance but since you said romance being a sub plot is fine I will add this one in
This show is currently airing and is about a Romance between a Witch and a Crown Prince. So far it has been good so you might want to check it out
This is amongst my personal favorite romance animes. The main couple is in College and even the side couples are all Adults (some in college, some working). Its a shoujo anime tho so you might not like it if you aren't into that type of stuff
Its another cute romance series where both of the MCs are adults. It also has a few elements of fantasy and action so you may enjoy it a lot
This last one is another adult romance series that I personally liked but I am not sure if you are into stuff like this so I am just leaving this here at the end
Cherry Magic Thirty Years of Virginity Can Make You a Wizard
There is not a thing such as adult romance in anime lmaooo (well, maybe 2-3 tops)
Grow up already and watch the Before Trilogy.
You've freaking got A Sign of Affection in your favourites what are you talking about–
You don't think there could be a lot more too? Considering the amount of romance anime that exist and how you know there is at least one?
C'mon, A Sign of Affection is high school level romance.
Did you notice how Yuki freaked out when she was alone with Itsoumi in his room? That's some 13 years old first crush reaction.
Did you notice how everyone and everything is super pretty and cute?
That shit is amied at unexperienced little girls.
The fact that something is set in an adult environment will not automatically make it "adult".
This is all I would consider "for adults" out of the 120+ romances I have watched so far.
https://myanimelist.net/anime/53585/Modern_Love_Tokyo__Kare_ga_Kanaderu_Futari_no_Shirabe
Focused on romance:
A Sign of Affection
Princess Jellyfish
Snow White with the Red Hair
TONIKAWA Over The Moon For You
Romance sub-plot:
The Great Passage
Plastic Memories
Yona of the Dawn
Just one franchise, but probably worth it:
WHITE ALBUM
WHITE ALBUM 2nd Season
WHITE ALBUM 2
You may want to watch 2nd Season and 2 also.
The beginning might not be that good but eventually, as the story goes on, it will be better and better - also beware of some indecency.
I present to you~
Josee to Tora to Sakanatachi