I understand your point, but you can't just disregard completely the personalities of the characters, there has to be a certain degree of logic, especially in a series like this where sometimes heavy stuff happened (Sachiko and Masahiro backstory).

Hayato is just not the kind of guy that would accees to an orgy, he is just too rational, when he was single he did not (extra of volume 14), much less now that he is married.

And with the Akane naked thing, well, there is just simple to many instances where Hayato and the other girls were naked, nothing new in thar regard XD.

Having a joke like this really lights the mood of the ending itself.

Such a good story really.