But you're not even using realism in your arguments for realism. Species are nothing more than a human created classification system for scientific purposes. Making the claim that a species in a fantasy universe can't deviate from what is expected in an artificial classification system is disingenuous if you know and understand reptilian biology. Different evolutionary paths could quite easily result in dragonborn with dragonboobs. Evolution doesn't conform to a set of guidelines. A species will either change or die out. It just so happens that dragonborn in the D&D universe evolved boobs.Well my knowledge comes from breeding reptiles and while I know that there are some that don't follow the standard of laying eggs and not nursing most do as the same with mammals the main definition of a mammal is a warm blooded animal that gives live birth and nurse its offspring and yes there are exceptions.
Half the discussions we have here could be classified as silly since we are dealing with a fantasy game. But that is part of the enjoyment having discussions like should dragons have boobs.
Personally I hate the argument that you can't have realism in a fantasy game it is old and trite.
Or they didn't. Honestly, I don't care. Short of needing a picture for your character, what difference does it really make? Don't like dragonboobs, say your female dragonborn doesn't have boobs. Boom! D&D magic.