Not all mages are sterile.
There's a quotation at the beginning of chapter 7 in the Blood of Elves:
No one is born a wizard. We still know too little about genetics and
the mechanisms of heredity. We sacrifice too little time and means on
research. Unfortunately, we constantly try to pass on inherited
magical abilities in, so to say, a natural way. Results of these
pseudo-experiments can be seen all too often in town gutters and
within temple walls. We see too many of them, and too frequently come
across morons and women in a catatonic state, dribbling seers who soil
themselves, seeresses, village oracles and miracle-workers, cretins
whose minds are degenerate due to the inherited, uncontrolled Force.
These morons and cretins can also have offspring, can pass on
abilities and thus degenerate further. Is anyone in a position to
foresee or describe how the last link in such a chain will look?
Most of us wizards lose the ability to procreate due to somatic
changes and dysfunction of the pituitary gland. Some wizards -usually
women – attune to magic while still maintaining efficiency of the
gonads. They can conceive and give birth – and have the audacity to
consider this happiness and a blessing. But I repeat: no one is born a
wizard. And no one should be born one! Conscious of the gravity of what
I write, I answer the question posed at the Congress in Cidaris. I
answer most emphatically: each one of us must decide what she wants to
be – a wizard or a mother.
I demand all apprentices be sterilised. Without exception.
Tissaia de Vries, The Poisoned Source