Rebuttal
From a real-world standpoint, it's true.
But by this hard-core biology there can be no humanoid race other than humans and anything that is clearly a primate. And it couldn't be over a certain height and keep a very human appearance (bone and muscle structure). So no lizardfolk, centaurs, kobolds, giants, etc..
And then the most agressive, prolific, and adaptable race would spread so fast and kill off most of the other erect primates.
I agree for the most part with your assesment—the threshold dividing the conditionally believable fluff from the outright cheesy can vary for each individual and subject, but it
does exist, and Wizards has arguably crossed it with the Dragonborn.
Boobs are generally great on the mammalian humanoids. (I Don't think many will argue with that.) However, the placement of breasts on a creature that has no obvious evolutionary purpose for them taxes the suspension of disbelief, which in many cases can be critical to enjoying a fantasy/sci-fi experience. Now, I understand that an argument can be made to take the fluff at face value and say that the race sprung from the blood of the dying Dragon god Io, but if you are going to take that stance, then you have to admit that Dragon's aren't mammalian and thus not monotremes,and they
darn well don't have boobs. Personally, I think it is unlikely that any dragon or (especially) dragon-god would have any use for or want to have anything to do with breasts. If I were a dying dragon god, my last act as a celestial being wouldn't be to make my begotten-not-made descendents top-heavy with useless fleshbags.
Now, take as an example the 3e Half-Dragons. This race had a logical explanation for the boobage. As half-human (or half-whatever else,) the breasts were assumed to come from the non-draconic side of the character's ancestral tree. This was a
much easier pill to swallow. Likewise, the fluff explanation for the intermingling of dragon blood with the general population of DnD races was not only believeable, but
Awesome. It added more reasons for Dragons to become involved with the affairs of the other races and contributed to the Dragon's characterization as an NPC character archetype in the campaign. It also hardly even stretched the suspension of disbelief—If I were cooped up in a lair for centuries without companionship, I'd be
horny as 
.
The fact that Wizards felt it necessary to justify their pro-furry pandering with this ill-conceived monotreme retcon speaks to the fact that
dragonboobs just dont wash with the geek community. Wizards' decision to can a rich and perfectly workable backstory for the Half-Dragon is emblematic of everything else that is wrong with 4e: they overreached and threw out much of what worked about 3e and gave that edition its characteristic flavor. And if they are going to try and sell me a subscription to the fluff through Dragon? That's outrageous. Anyone not subscribing to DDI will automatically be getting an impoverished gaming experience, regardless of the quality of the fluff they release in the future
Also I think that I would be negligent in my duties if I did not reiterate that we are
not furries:
http://hackvan.com/pub/stig/funny/geek/the-geek-hierarchy/the-geek-hierarchy-chart-2.0-abridged.gif
The spirit of the game is whatever we wish. The spirit of my game is FUN. If you're having fun, who is anyone else to tell you you're "doing it wrong?" You have that authority?
The spirit of the game is whatever my DM wishes. If my DM decides that Orcs or Tieflings came from the planet Xanadu in fleets of mighty spaceships made of cheese, as the player I'd be forced to eat
and like it. Only I wouldn't, and I'd be really unhappy because that kind of tampering with community-established standards for a fantasy realm disrupts the suspension of disbelief requisite to having a good time adventuring through fantasy realms. In any case, it is overreaching on the part of the DM. As an RPGA member, my DM is Wizards. He can be a real dick.
Dragonborns being added to D&D somehow makes it more like a Micheal Bay movie? I guess, if you picture the dragonborn's fight scenes in jerky, out-of-focus snippets. When I picture dragonborn I get visions of Braveheart, Predator, Conan, The 13th Warrior... perhaps they're shallow to you but it seems you made your mind up long before trying to imagine a brave, heroic dragonborn paladin (for example).
GOD. Transformers was the WORST MOVIE EVER.
On-topic though, I was referring more to the over-the-top style of the new fluff that wizards is pushing on us. Others have likened it to a WoWification of DnD. I know that i am not the only one who prefers the intricate fluff of the default 3e setting as opposed to the broad and garish cartoon strokes that are taking place now with 4e. I mean, if I wanted cartoon action hero, i would play in the Eberron campaign. Wasn't that why they made the Ebberon setting? There had to be some niche reason for it.
Dang, so you're sitting outside with some other players, the baby, and the bathwater? I figure your time would be better spent with a system you enjoy.
I would, but the RPGA is strictly Wizards stuff, and the announcement of 4e was the death knell of Living Greyhawk. They're pulling the rug out from under us and forced early retirment for all our 3e characters. (I JUST got access to the Drunken Master prestige class too. *Grumble*)
As it stands, I don't have a schedule that permits weekly get-togethers with a DM and a group of friends. Personally, I like to game as god intended: in sleepless, uninterrupted 72 hour caffeine-fueled stretches at cons with a bunch of other likeminded gamers. For me that means I'm stuck with 4th Edition, but I am not going to be silent about it.
I imagine that a bunch of LG fans are as angry about 4e as I, but I sincerely hope that they don't stop coming to cons, because I still want to play.
(To avoid just saying "LOL") 
