多くても双子ぐらいがボリュームゾーンのヒトにとって乳首の数が2個なのも合理的である。しかしオスにも乳首がついているのは非合理である
Discussion
It's probably about cost.
The cost of differentially adding or removing nipples based on sex of the foetus in the womb..it may not be worth the effort to change if there's nothing too adverse happening because males have nipples => There's no advantage to removing it biologically speaking + it has a cost to make the change