They're only the same species as science in the real world defines them, and as we know, science in the real world breaks down when applied to D&D in many scenarios.
The Frankenstein's Monster mode of creation flies in the face of all of the existing "descriptions" for how half-dragons come to be. They say that a dragon "mates" with another creature, not that a dragon creates an abomination to life and science with another creature...
Yes, its really easy to write that, because what the writer wants is dragon/human hybrids. The problems are legion.
Dragons have cloaca. How the hell do they mate with humans?
who says? just because the red dragon's junk isn't hanging out in the MM doesn't mean it's got a cloaca, any more than the lack of pegasus junk gives them cloaca, it merely means the artist didn't feel like being realistic enough for the book to require a mature warning.
Dragon sperm isn't going to be accepted by a human egg. Human sperm isn't going to be accepted by a dragon egg.
why not? seriously, give me a reason. "It doesn't work that way in real life" doesn't cut it for me since we don't flying fire breathing quasi-reptilian creatures running around in real life.
Dragons lay eggs, humans give birth live.
so what? No, really, so what? It's not that humans don't have eggs, they do, they're just not eggs in the colloquial sense. And don't forget, there are snakes that give live birth and mammals that lay eggs. Live vrs. Egg birth is not a dividing line for reptiles and mammals. I have no clue where I'm going with this...
Dragons have a rather different cranial arrangement with humans that is utterly incompatible, including, among other things, the ear bones of a dragon are fused to the jaw (which leads to them being differently located about the skull as well), the musculature for working the jaw is arranged differently (don't have the right temporal fenestra), they have two temporal fenestrae instead of one, and neither of their temporal fenestrae are homologous to the mammalian one. I could go on, the list of differences is rather exhaustive and would probably take pages.
again, so what? This means nothing when it comes to half dragons, as half dragons are magically created creatures (whether through natural magic or artificially induced magic). If the half dragon has a draconian head, he'll have a draconian skull, if the half dragon has a humanoid head, he'll have a humanoid skull.
Dragons have a three-chambered heart, mammals a four-chambered heart.
*pulls out draconomicon* canon says a dragon has a four chambered heart just like a mammal.
Teeth differences - dragon teeth are undifferentiated cutting blades. Mammalian teeth are highly specialized. An ad hoc intermediary is going to be non-functional.
Dragons are Omnivores, in the truest sense of the word, I'd say they probably have some highly specialized teeth as well, which could well be adopted by a half dragon.
Scales vs. skin
human genetic disorders say
"your point?"Dragons don't have sweat glands, mammals do.
Dragons don't have to sweat, their fundamentum uses excess body heat to fuel the breath weapon, which half-dragons have also.
hexapod vs. tetrapod
er, sorta.. but anyway, what does this have to do with anything regarding mating? If you're thinking positioning...
Cloaca = urinary, intestinal, and genital tract in one tube. Mammals partition these into three tubes. (two of them happen to rejoin in males, but they are distinct tubes within the body prior to that even then).
hmm... Draconomicon didn't go into any detail regarding reproductive structures... I can't imagine why, I mean, they explain Beholder and Illithid reproduction... and beholder reproduction involves roman showers...
(it just struck me how appropriate that smiley is for that line...)
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And that's an abbreviated list.
Seriously, 'magicking' that away is going to take deliberate effort and a guided purpose. You can't just handwave it away.
er, that's the arguement Creationist and ID people use against evolution...
incubus cannot successfully mate with a balor (or at least we have no evidence to the contrary).
the Half-fiend template can be applied to fiends, so, yes, an incubus can mate with a Balor. Though not most devils, but that's for completely different reasons.