So it wasn't that he was simply limited to anal.
It's that he's literally restricted from willing sex. He needs to have it occur through some stupid lucky coincidence slapstick comedy sex which realistically would completely destroy someone's chunchunmaru from the impact of an entire body hurling at top speed from the sky unto your lap.
It's a literal miracle his blade hasn't bent and broken from that sort of impact. Truly a legendary sword.