Does the “protection” instinct have anything to do with a primal instinct to protect a baby’s food source? That would explain the cover-it-up aspect. This, I guess, could be naturally extended to society going along with it and gradually tabooing the female breast, and in particular, the nipple…
Over the years, as other techs have come along, negating the need for protection, the societal sigma has remained in place, keeping the status quo of the case of the banned tit.
This would also explain why men have no reservations about nipps showing; nothing to protect, nothing to be ashamed of, nothing ever came of it…