Nipples are not a sex-linked characteristic. In other words, nipples are just one of those sexually neutral pieces of equipment, like arms or brains, that humans get regardless of sex.
Every human being gets a unique set of 23 pairs of chromosomes at conception. These fall into two categories. One pair of chromosomes determines sex - the XX combination means you become female, the XY combination means you become male.
The other 22 pairs, the non-sex chromosomes (autosomes), supply the standard equipment that all humans get. These 22 pairs constitute an all purpose genetic blueprint that in effect is programmed for either maleness or femaleness by the sex chromosomes. The programming is done by the hormones secreted by the sex glands.
For example, the autosomes give you a voice box, while the sex hormones determine whether it's going to be a deep male voice or a high female voice. Similarly, the autosomes give you nipples, and the sex hormones determine whether said nipples are going to be functioning (in females) or not (in males).
One interesting consequence of the developmental set-up just described is that during the very early stages of fetal life, before the sex hormones have had a chance to do their stuff, all humans are basically bisexual. Among other things, you have two sets of primitive plumbing - one male, one female. Only one set develops into a mature urogenital system, but you retain traces of the other for the rest of your life.
It's tempting, therefore, to say that male nipples are yet another vestige of your carefree bisexual youth. Male nipples though are not vestigial. They're full-sized and fully equipped with blood vessels, nerves, and all the usual appurtenances of functioning organs. .. Both male and female babies are born with the main milk ducts intact - the gland that produces milk is there in the male, but it remains undeveloped unless stimulated by the female hormone, estrogen.
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