i) Darwinians sometimes taunt creationists with the question: Why do men have nipples? They seem to think that's inconsistent with creationism. Perhaps they think Christians are embarrassed by discussing nipples.
ii) To begin with, it's not as if there's a good evolutionary explanation. Male nipples have no survival value.
And it makes no sense to say male nipples are vestigial organs. Even on evolutionary grounds, it's not as though men evolved from mammals that were exclusively female. Even from an evolutionary standpoint, mammals were always sexually differentiated. For that matter, so are reptiles, from which mammals allegedly evolved.
iii) To my knowledge, the reason men have nipples is because men and women share the same basic underlying design. Our bodies have most things in common. Engineering is conservative.
Sexual differentiation is due to sex chromosomes and male or female hormones. But that leaves many underlying structures intact.
Even in sexually mature adults, if you administer male sex hormones to women or female sex hormones to men, they develop some characteristics of the opposite sex.
iv) Sexual arousal is based, in part, on touch. Because male nipples have nerves, that's an "erogenous zone." So it's not useless.
v) It's sometimes said that all humans begin as female in the womb, but that's simplistic. The male or female DNA is present from the get-go.
vi) Finally, lactation is a remarkable process. We think of animals as food. And trees produce food. But for animals to produce food is rather remarkable, if you think about it. Women have a whole little factory for producing a vital food stuff.