There is plenty of evidence that women slaves were preferred to men because of their role in household tasks, and especially as wet nurses. Especially in the Iberian Peninsula as opposed to elsewhere in the Mediterranean, wet nurses were often slaves.
Because of Ramon Llull's ideas—and of course the tendency of human populations to despise those who are different—these Muslim slaves were often forced to become baptizata, "the baptized" (converts from Islam), so that they would be proper wet nurses and caregivers for the children.
This relationship could be beneficial to the wet nurse, besides the opportunity to be treated well while they had primary "control" over the newborn. In 1266, a Barcelonan patrician named Romeu Durfort left his baptizata wet nurse a legacy of 40 sous and charged his heir with maintaining her for life. In 1280, a burgess freed her baptizata, Esclarmonda, and all Esclarmonda's children in gratitude for Esclarmonda's wet nursing.
This is not to say that life was wonderful for the Muslim-born mother:
In this society elite Christian mothers were to bear legitimate children and enslaved Muslim women who bore children were urged to convert, watch their masters send their infants to be nursed by someone else, and then serve as wet nurses to the heirs of their masters and mistresses. [link, p.169]
Llull's work on raising children also shows a distrust of "immoral servant women" and warns that mothers should not leave their daughters at home with the servants when the mothers go out. Llull assumes hostility between the wife and her household servants.
For the Muslim-born population, whether they were servants are free, breastfeeding the child of another created a situation referred to as "milk siblings" or "milk kinship." We will take a closer look at the Koran's views on wet nursing tomorrow.