My dear Kiredor.
Why '
female Orcs' are never expressly mentioned I do not know.
But we do read this in Silmarillion,
Quenta Silmarillion,, Chaptere 3, "Of the coming of the
Elves and the Captivity of
Melkor"
"But of those unhappy ones who were ensnared by Melkor little is known of a certainty. For who of the living has descended into the pits of
Utumno, or has explored the darkness of the counsels of Melkor? Yet this is held true by
the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous
race of the
Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest foes. For
the Orcs had life and multiplied after the manner of the Children of Ilúvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the
Ainulindalë before the Beginning: so say the wise. And deep in their dark hearts the Orcs loathed
the Master whom they served in fear, the maker only of their misery. This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar."
The Children of Ilúvatar are of course the Elves,
Men and
Dwarves. (The first two being the Children of his choice, the last being the Children of his adoption. (Quenta Silmarillion Chapter 2: Of
Aulë and
Yavanna) - "But when the time comes I will awaken them, and they shall be to thee as children; and often strife shall arise between thine and mine, the children of my adoption and the children of my choice.")
So there must have been female Orcs. Perhaps they looked so much like the males that it was hard to distinguish the difference.
I'm not sure I ever specifically read
Tolkien saying "male" Orcs either, so perhaps that they are all "male" is an assumption on our part?!
That's the best I've got. There are many better educated than I am here, so maybe they can do better.