Lord Sessadore
Explorer
Does it really matter that much? I mean, do round things on the chest of female humanoids really call for that much analysis and criticism?No they don't. They don't HAVE milk. That was added after the fact.
Timeline:
Dragonborn have breasts. People go "Uhhh what. They're lizards that lay eggs."
Dragonborn are told to have milk. People go "Uhhh what. They still lay eggs."
Dragonborn are now apparently platypi. People go "You really don't know how this works, do you?"
I really think it's unwise to apply real-world biology and genetics to fantasy. I really do. I mean, we're talking about dragons, for Pete's sake. A creature like that simply wouldn't work with "real" biology. The people going "Uhhh what?" in your timeline were, for the most part, people who wanted to apply real-world science to D&D. There were also a lot of people who never gave it a second thought, or just said, "Yeah, whatever. They've got boobs. So?"
But if you really need a "plausible" explanation, then just imagine that female dragonborn have extra-large pecs or extra fat stores in that area because male dragonborn also find boobs attractive. Or maybe they're just mammals with scales on. It's only a big deal if you choose make it one.