To OP.
It do make sense that peasant women is only possible to get from prisoners or knocking them out and recruiting them from your prisoner list.
Why? In the medieval times women were rarely soldiers, those few who were often people of plundered villages that lost their husband and kids, who then sticked around the army/warband/whatever that saved them, possibly picking up some training, etc.
(not directly historic knowledge, but a theory based on the attitude towards women in the middle ages)