Over time, their bodies evolved for two reasons.
To survive and to reproduce!
The males evolve to develop a powerful body, with great stamina and vitality. This allowed them to survive harsh climates and survive the deadly battles against their many enemies. Not only this, but their bodies adapt during sexual intercourse. They could create a light aphrodisiac effect to increase the pleasure of their partner.
Paired with their superior bodies, they became a race that males from other races despised and saw as monsters.
While the females evolved in a similar direction...
They were special, and any man who mated with a female orc could never forget their special traits.
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