They are couple.
The girl is playing the shamisen.
(a Japanese traditional instrument with three strings played like a guitar)
The boy is singing with her shamisen.
Maybe they were excited to play and sing a song.
It changed to sexual excitement..
They can’t stop to do it.
—
Furyu juniki no eiga
Koryusai Isoda
1773
風流十二季の栄花
礒田湖龍斎
1773年