This been bugging me for years.
It's about an adult couple living together, and there's also a secondary couple.
The secondary couple consists of schoolgirls with typical butch/femme dynamic (I think, can't quite remember).
The more feminine of the secondary couple is either living together with or like to visit the main couple's house. She might be a relative or a neighbour, and the main couple often acts like mentors for the younger girls.