I would opine that it would be a subjective call on the part of the SG, having read through the Becoming ritual and seen nothing explicitly forbidding it.
One school of thought would contend that IF the maga/magus has already undergone the longevity ritual in human form before the Becoming, then they were already strerilized and tough luck for them.
Another school might counter by saying "Yes, but the Becoming explicitly states that it changes the Mind, BODY and Spirit of the Maga/Magus and that in doing so it eliminates the need for any further longevity rituals. Thus with such dramatic physical alterations inherent to becoming a creature of "popular imagination", any human physical limitations should no longer apply (even those resulting from magic).
A third school of thought (to which I would be inclined to subscribe) would simply say, "How daft would a maga/magus have to be to ever conceive of being a Fairy in the first place??" 
(Ok, maybe you become a raving lunatic, but in my case, who'll notice? 
)