male nipples are developmental vestiges, the male condition is derived from response to embryonic testosterone, and is a developmental variation from default.

early stage embryos of both sexes are not easily distinguishable by genitalia, they look morphologically similar. later developmental events culminate in morphological rearrangement to male form.

lack of response to testosterone during development results n a curious state of affairs, where a person is genetically male, having x, and y chromosomes, develops according to a female plan. external appearances are female, with loss of secondary sex development in puberty.

Androgen insensitivity syndrome:

https://en.wikipedia.org/wiki/Androgen_insensitivity_syndrom...