This series is character of the week with a loosely connected plot and minor Nanako subplot so harem ending was the best way for it to end. It could have been better with interactions between other girls in the club but it was never structured that way.

My headcanon is that the other girls do have sex with each other, but that they won't if they get together with Miyako.

That's an awfully big harem though. I hope Miyako stays in shape!

They'll be keeping her in shape.

man the time management must be tight for the girl just to meet miyako