Kabe no Mukou no Tsuma no Koe F ~Katsute Tawamure Atta Oretachi wa, Kousaku Suru In'en to Tomo ni Nikuyoku e to Obore Yuku~

Yuuto Tokunaga and his wife, Satsuki, find themselves with new neighbors: Koudai and Ria, a married couple whom they're also friends with.

Having gotten along since their student days, the four of them begin drinking together and rekindling their old bond.

One night, Satsuki made a candid, drunken confession: "To be honest, Koudai, I used to have a crush on you back in our student days."
The delighted Kodai replied, "Actually, I was interested in you, too."
Ria then followed suit, revealing, "Truth be told, I had a crush on Yuuto."
Flustered, Yuuto admited that he, too, had liked Ria.

"So, why don’t we all kiss each other?"

Ria suggested, perhaps spurred on by the alcohol.

Satsuki agreed with her, then kissed Koudai. Yuuto watches the two of them kiss, unable to stop himself. Ria then kisses Yuuto, and as they caught glimpses of each other kissing, the excitement among the four of them mounts.

And from there, things naturally progress into wife swapping...

Afterward, they discuss whether to continue doing this. Having experienced the thrill of swapping, everyone agrees to it.

Yuuto, Satsuki, Koudai, and Ria—
The four of them begin their lives as swingers.

[Translated from website]