Although all the races of the Whorl (except the Forged) derive from humans, the techniques used by the various gods to create them leaves them incompatible and infertile should they come to love members of another race. One of the two exceptions to this appear to be Humans and Orcs, one of the many varieties of monsters created by Ravana during the Godswar.
Orcs are not the largest, strongest, nor most cunning of the monster-races, but they are the closest to baseline humans. This may explain why, of all the potential interbreedings over the last few millenia, only orcs seem capable of creating children with humans.
Although one might be forgiven for assuming that all half-orcs are the product of rape, this is not so. Although many such tragedies do, in fact, occur, nothing about orcs makes them intrinsically evil or hostile to the other races. Indeed, of all the races of Ravana derogatorily called "monsters" the orcs are the most human in temperament as well as physicality. Some brave the stigma and social slings and arrows of civilized society, where they fall in love, make friends, or have children as any other person would.
Still, half-orcs do inherit traits from their orc side including a more robust physique and a certain savagery in combat.