huh? I didn't realize that one fiction genre had stircter rules than another. Since it's all....... fictional.
Granted people have different opinions on how fictional ideas should function.
The best break down of science fiction vs fantasy that I've seen is:
Science Fiction is the fiction of what could happen
Fantasy is the fiction of what can't happen
Science Fiction tends to be speculative and try to stay within the bounds of science.
Fantasy tends to be metaphorical and tends to make its own bounds.
X-Men, Spider Man and Superman, for example, are largely Fantasy rather than sci-fi, even if the cloak their magic in psuedo-scientific babble and terms like mutants and aliens.
Batman and Iron Man, however, taken on their own are largely Science Fiction. Though Iron-Man starts to cross the line with what his suit can do.
There is some bleed over, especially when you reach the level of technology that might as well be magic, but, for the most part that is the difference.
In a hard science-fiction, two separate species, even if built on the same basic theory of reproduction, would not be able to reproduce in most cases unless they are very closely related species. Now, this is not to say that hybridization is not more common than we think it is. Snakes can successfully interbreed (a real potential concern given a flood of big constrictors that got released in Florida by a hurricane a few years back). Bees have interbred (killer bees anyone?). And not all of the liger and tion offspring are sterile. Still, it is difficult. Note that Spock in Star Trek was apparently produced with a great deal of help from Vulcan and Earth scientists. (Next Generation got a bit looser with producing hybrids than the original series did).
By contrast, in a fantasy, interspecies romances will work if the author says they'll work.
For example, in most of my works, there's no difference in my eyes between the soul of a human and the soul of a demon. Maybe it's the God-Forgives-Christian in me, but I tend to operate under the assumption a soul is a soul is a soul and that we're all essentially related through that one fact and that the workings of our bodies are merely the expressions of the assumptions and beliefs of that soul.
If a fantasy story has need of a hybrid character, then such a character exists and such a character is possible to exist. The possibility fits the need of the story and you have a much easier time explaining it believably.
Now, what the mixed blood is a metaphor of, if anything, depends on the individual reader. Meaning like that might be something the writer thinks he has a say in, but the meaning the writer sees is the meaning that makes sense to him or her given their own life experiences to that point. A reader with a different set of experiences gains a widely different view on the matter