Huh, there was a 4-part hentai series that was all about that
Don't remember the name of the manga though
All I remember was the first four parts were amazing and the fifth part was crap
Oh hey, I found mine:
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I really liked it overall, but the fifth part is still BS. Mostly because the first four chapters are so good. They're all about the couple learning to love and trust each other in their relationship, and the fifth part is all like "Hey, let's just throw a second girl in there for no reason other than to sell more copies, story be damned."