The ability to breast feed its young is the single-most defining trait in a mammal (scientifically speaking). Having scales, laying eggs, etc., are all traits common in non-mammals, never-the-less there are mammals with many of these traits (take the platapus as an example).
To argue that dragonborn have true breasts is to argue that they are also mammals (since there is no such thing as a mammal that doesn't breast feed its young).
Dragons have been classified as warm-blooded creatures in D&D (and therefore aren't reptiles), but neither are they mammals since they don't breast feed their young. If dragonborn share the traits of true dragons (call it genes or whatever else you want) than, like their true dragon cousins, they should not have breasts, should not breast feed their young, and should not be classified as mammals.
