"From conception until sexual differentiation, all mammalian fetuses within the same species look the same, regardless of sex. In humans, this lasts for around 6 weeks; after which genetically-male fetuses begin producing male hormones such as testosterone. Usually, males' nipples do not change much past this point..."
"... Their (nipples) advantage in females, in terms of reproductive success, is clear. But because the genetic "default" is for males and females to share characters, the presence of nipples in males is probably best explained as a genetic correlation that persists through them, rather than selection for them...."
these are quoted from wikipedia. the answer is pretty much what i expect.