Aru Tsuma no Natsu no Jouji

Mr. and Mrs. Tsukishiro rent a mountain cottage for their husband's summer vacation. They meet a local boy named “Fujikawa Shosuke” by chance and ask him to show them where they can swim in the river.

The wife, Yurie, is pressured by her husband to go swim in the river, which is said to be a deserted place.

She is forced to develop her own anus without her husband's knowledge, and her unforgettable day begins as she falls into a state of immorality.

[From DLsite]