Before the Sundering, it is said that the various humanoid kinds were so distinct that cross-breeding was impossible. Something in the degeneration of the world after it shattered has changed that. Now, on rare occassions, cross-kind breeding does take place. In every instance these half-breeds are sterile, incapable of having offspring. In every instance except for Half-humans. Something within humans allow them to have fertile offspring with most of the other kinds on Loar. These offspring can only then successfully mate with other Humans or with whatever the other kind the parent happened to be. Human-Orc half-breeds cannot successfully mate with an Elf and produce fertile offspring. This causes the various humanoid kinds to avoid romantic relationships with any but Humans.
Half-breeds in society are accepted. Those rare instances of non-human half-breeds are pittied at best. Most lack any connection to the elemental realms and do not become spell casters unless they are intelligent enough to become wizards. In many cases, the half-breed population is seen as a huge tragedy.