Watashi wa, Kairaku Izonshou

Based on erotic manga by Yabitsu Hiro.

A dude has recently moved into an apartment complex with his pregnant wife, when he accidentally stumbles upon an orgy going on in one of the rooms. He decides to get in on the action since his wife ain't been fucking him lately. He fucks one of his teenage neighbors to get things started, and ends up fucking his landlady for good measure.

(Source: Hentai Haven)