I think the romans just did what most slave-owning cultures did: they removed the source for trouble in their slaves. They knew very well that the only way to effeciently run slave business was to emasculate the male servants. After all, you geld stallions and bulls too, don´t you. Sometimes it was the mistress of the house who ordered the surgery. There was, for example, one rich woman in alexandria who had her male slaves castrated because she could not stand the sounds of masturbation coming out of the slave rooms at night...