[Galadriel55]

It's possible that orcs used slave women to multiply. Or they could use the women of Khand, Harad, Rhun, and the other places that were under the dominion of Sauron. It could be part of their agreement that these countries would send a certain numer of women on a monthly basis, or something.

I find a few drawbacks to this theory, though.

a) How do they keep orc features? Crossing a human with an orc will give you something like Ferny's southener, or at least like the Uruk-hai. But if an orc is only 1/32 orc and the rest human - wouldn't he look human?
OK, so it says that they add filth to keep the appearance, but wouldn't the character also change? Become something more honourable?

b) How do they have enough women? I don't think there are enough slave women in either Mordor or Isengard to triple the army's size. And they'd have to depopulate the surrounding kingdoms of females if they get the women from aliances. All women die after giving birth to an orc, so they need thousands of women. Where do they get them?

There was something else, but I can't remember.

The argument I have for Bolg son of Azog is that they were goblins. I think that goblins have a different origin than orcs, and they might breed differently.