Futago Niizuma Haruka to Setsuna no Momoiro Shinkon Seikatsu

Everyone knew Kazuki loved two women - twins to be exact - and they fiercely loved him in return. When the trio graduated from high school together, people began to wonder who he would marry. No matter whom he choose, Kazuki would never truly be happy, and the heart of someone he held dear would be devastated. Then a miracle happened: polygamy became legal. The true answer found, Kazuki entered holy matrimony with both sisters, and the pair happily accepted. It had always been the three of them; and now it seemed it always would be.

[From an old review by Nargrakhan]