technically, the nano-bot thingies that Steele has could be actively preventing it to prevent horrible mutant babies
It's not necessarily the How of it, but the Why of it. Considering the setting, the How of it could be any manner of magical science one would like to imagine. Quite frankly, the How of it could simply be that no one is particularly of a mind to write it.
ALSO considering this setting, however, and as to the Why of it, with any number of other was to impregnate, be impregnated, and subsequently rewrite the genetics of the offspring, would reasonably imply that a casual dismissal of it would not be especially internally consistent.