The men of the Orc tribes can only reproduce males so in order to continue the species they must have female slaves. No matter the race of the slave the baby born is always a male orc. The only exception being if a woman is pregnant. If she is already pregnant the child will be half orc and half whatever race the woman was impregnated by.
The impregnation of slaves is very ritualized with the slaves being garbed in white robes and led to the foot of an alter with the symbol of the one eye hanging on the back wall. For the slaves it is considered the best possible life to be chosen as one of the pure carriers. During pregnancy they are treated like queens and even after are held in special regard as mothers to the Orc race.
This is just the rough draft of what everything is like in Orc society. More details will be given when they pop into my head I promise. Leave a comment and a like or dislike.