In my travels around Outland, I've had the misfortune of encountering far too many fel orcs, whose flame-red skin, enormous tusks, and featureless eyes immediately distinguish them from the orcs to whom I have grown accustomed back home on Azeroth. (Strange that I now consider two whole continents to be "home"; how my world has grown.) In our repeated mutual murderings, it's come to my attention that I can't recall a single one of them being female. During my assault on Hellfire Citadel, I discovered that most of them seem to be cloned in tanks instead of natural births, so perhaps they really are nearly-all male.
However, this would not be the first case of orcs displaying something other than the two-gender biology to which we are accustomed. There is the outstanding issue of peons—I guess they could be a form of pre-pubescent orc, with the biological change into adulthood leading to increased body mass, assertiveness, and intelligence, but that doesn't seem to mesh with orcish cultural attitudes towards peons. Cultures tend to treat their young more protectively than the orcish warrior caste (in the generic sense, not the WoW class) treats the peons. Further discrediting this theory, peons seem to be fairly neuter in terms of gender. While it might be interesting to have a species that sexually differentiates late in life upon entering the warrior caste, the presence of gender-differentiated orcish children in Orgrimmar rules this out.
Thus, I think being a peon is for life. The question, then, is are they a caste or a morph. By this I mean, does peonism breed true? Are there effectively two orc subspecies that live together but don't interbreed, or can peons be born to warrior caste parents? I personally think the latter is more likely, given how peons seem to be neuter. I suspect they are incapable of reproducing and are effectively a third gender of orc adapted for manual labor. Their decreased intelligence is actually beneficial to them, as it keeps them from getting bored while doing work that is necessary for the tribe. In this case, I don't think there would be any stigma attached to a pair of orcish parents who give birth to a peon, as it would be recognized that they are a vital part of society. Further speculating, perhaps orcs are always born as twins, with one being warrior caste and the other a peon, the product of a single zygote that splits asymmetrically (not unlike the movie Twins with Arnold Schwarzenegger and Danny DeVito).