Nipples in male mammals is an example of evolutionary nonconstraint. In natural selection, sexual dimorphisms (differences) are the exceptions -- not the rule. Exceptions occur when constraints are required by natural selection, not just because something is unnecessary (like nipples on men).
Males and females share common genetic traits by default. So, although a trait in mammals may be as worthless as "tits on a boar hog", that trait will be shared by both sexes (genetic correlation) unless natural selection has a reason to eliminate it.
Many think that all babies start out female, but this is a myth. Their sex is determined by the male's sperm at the moment of conception. All developing babies start out with external sex organs that look female. If the baby is male, the internal sex organs mature and begin to produce the male hormone testosterone. If the hormones reach the tissues correctly, the external genitals that looked female change into the scrotum and penis.
Nipples, like most other parts of our bodies, are merely an example of genetic correlation. Now, the size of what's behind those nipples is a different matter.
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