Which is our mechanical reason for the setup.
Which is our legislative reason for ensuring certain lines of suitability are drawn. Yes, ratings are enforced by law in some countries.
The idea that nipple exposure may be a taboo subject sounds pretty on target to me. We're also playing in a society that, unlike our own real-life one, has absolutely ZERO discrimination between genders. (All 3 city-state leaders are currently female - Sultana, Elder Seedseer and Admiral. If anything, men are inferior in this culture.) So if nipple exposure is unacceptable for the ladies, it stands to reason that this would follow for the men as well.