Why on Earth? Male nipples
Why do men have nipples?
Because the basic human body-type is female. For the first ten weeks after conception, every human embryo looks the same, and the only way you can tell the difference is at the chromosomal level, where women have an XX chromosome pair, and men have an XY. At this stage, the only genitals that the embryos have are gonads. At ten weeks a boy’s sex-chromosomes leap into action, and his testes begin to form. They then release testosterone into the system, which triggers other changes like the appearance of male genitals, hair on face and chest, BO, football and so on. No testes, no testosterone, no maleness, and the embryo becomes a woman.
The point behind all this is that all embryos start off with nipples, and the appearance of testosterone in the system doesn’t mean that they disappear. In fact, in some XY embryos - men, in other words - the area of the Y-chromosome known as the SRY (sex determining region on the Y chromosome) can get damaged in some way and the embryo will develop as a woman, producing someone who is genetically male but biologically female.
There’s an interesting way of proving this theory. You may not know it, but the human body has a natural contraceptive built into it. When a woman is breast-feeding, it’s difficult for her to conceive another child, because having her nipples stimulated regularly sends neurohumoral and endocrine signals which stop her from ovulating - releasing another egg, in other words. No egg, no baby. The point of this little digression (apart from letting you know that playing with your girlfriend’s breasts may stop her getting pregnant) is that if you stimulate a man’s nipples, it’ll cause exactly the same neurohumoral and endocrine signals, designed to stop him ovulating - which makes no sense, because men can’t ovulate, even if they wanted to. Which proves the basic human body-type is female, nipples and all.
So there you have it. A man is a retrofitted woman with some extra bits bolted on. Have a nice day.