This is not SHUNGA actually.
The title of this painting is “Nebitori”.
“Nebutori” means the sleeping fatty.
Nebutori is a yokai disease. It only infects women, and makes them fat while they sleep–either suddenly or gradually. It is considered an infectious disease, like a bacteria. Women infected with nebutori don’t necessesarily eat more—they just get fat while they sleep.
Ehon hyaku monogatari