Well, the universe seems to have rather greater understanding of the brain than ours (witness Steve Rogers' serum, the memory implantation used by the Red Room, and the mindwipe tech used on the Winter Soldier - we have nothing like those), so it doesn't seem beyond belief that they could implement something that would give the end result of asexuality. Perhaps memories could be stripped/analysed from an asexual subject, and identical memories implanted in the Widow girls? Or, perhaps, they might have developed a medical test for the gene/brain configuration/whatever that would let them test their potential candidates; even if it wasn't foolproof, being able to raise the chances of a candidate being asexual could be useful to them.