Not sure this fits exactly, and when I say this is fucked up I mean it's fucked up, but I really enjoyed 4-episode series Kenja no Ai. It's about a woman who is in a relationship with her best friend's son. It's more of a revenge story than a romance story, though. We spend more time fleshing out Mayuko than her relationship with Naomi.
Kimi wa Petto, both the 2003 and 2017 versions, are not necessarily a tale of forbidden romance but definitely unconventional and one that other people wouldn't and don't understand. I would forbid it LOL. It's about a businesswoman who takes in a young man as a "pet" because he has nowhere else to go. She's sort of fucking with him when she suggests it, but he's like yes please thank you goshujinsama no takebacksies. They're very physically affectionate, which I liked.
The 2003 version does a better job at making the leads feel three dimensional, but the 2017 version is visually better and came out after the manga was finished so the end is a little less messy.
Milisia:
