No defined NIPPLES, buddy. Breasts are still fair game.
Discernible breasts are rare enough on animals with nipples. In a monotreme, there is no equivalent structure. The young basically lick milk off their mothers.
The closest genetic relatives to humans are chimpanzees and gorillas. Have you ever seen ape cleavage? I'm going to assume the answer is, "No." If apes don't have them, if cats don't have them, why would something as marginally mammalian as a monotreme have them? That would be like arguing deer people should have venomous antlers because male platypi have poison spurs on their feet.
Dragonboobs do not exist specifically for pornographic reasons, or to give female players an idealized avatar, or because it added something to their backstory for them to be mammals. They have dragonboobs because the design team thought it was the way to go. They are simply the result of the psychological unease some people feel if males and females are not protrayed as essentially different. Who cares if you can tell a male dragonborn apart from a female, apart from another dragonborn? If they really wanted some kind of dimorphism, they could have chosen crests, or coloration, or size, or any of the more usual differences in the animal kingdom.
I don't object to fantastic biology. I don't object to breasts. I don't even object to breasts on creatures that are reptilian appearance. I don't have a problem with breasts on half-dragons, medusas, sorceresses, or, for that matter, weird outsiders that look kind of like insects. I just don't understand why they would exist on creatures that are basically dragon-people. I could accept visible breasts on 3e-style dragonborn, who are members of other races that have been transformed.
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