Secret Garden by Amagatsu Toriko and Egawa Hiromi — Ongoing
This is a different manga from the similarly-titled one by Fujii Mihona.
This one bills itself as a "nonfiction manga" about a couple who are based on real people. Misono and Masami are working women in a long-term relationship; they even live together. Most of the story revolves around their sexual exploits, but there are also some sweet domestic moments.