Deep down inside, a sister knows what's expected of her, and what idealized RJ-created woman would seek to conceive & give birth to a kid she would have to put second to the Tower's interests? The Tower demands they swear the Three Oaths so that they don't get involved in worldly matters - how would it take an Aes Sedai refusing to participate in some scheme because her middle-aged grandson (who has become a figure of influence and power thanks to the generous stake his parents received as a wedding present, and the superior educations their family got through their Aes Sedai progenitrix ) in the real world will be adversely affected or even ruined by the Tower's agenda. What if this had not been the end times, and Moiraine had had children and 50 years later, she is one of the top dogs in the Tower, and close friends with a couple of influential Blues, and all of a sudden, the Tower's agenda will have bad repercussions for Cairhien and its aristocracy? How far will Moiraine go to protect her children and grandchildren, who would be Damodreds and almost certainly involved in the House's affairs? How do you sort out the tangle of one sister's sacrosanct personal interests? What sister can safely involve herself in the affairs of Cairhien without trespassing on Moiraine's "business"? With no personal stake in the matter, Elaida seems to have prevented much of the rest of the Tower's ability to place eyes and ears in the palace in Caemlyn - how much more insistant would sisters be on their rights to non-interference when it is their own offspring involved? We see that Elaida can be made to conform to the Tower's agenda for Andor despite it being her "business" but would any sister be so amenable when it is her kids, as well as her schemes and agenda at stake? For a scheme or manuever that fails due to preemption by Tower interests, a sister can shrug it off and try again another day. A couple of decades is not that much off an Aes Sedai's life in the long run - no more than five years from a normal person, and without the aging issues. On the other hand, the life or wellbeing of her child is a completely different story, no matter the similar investment of time. Few women are sanguine enough to shrug it off and have a new kid to replace the one murdered during a political upheaval, or assassinated in retribution for a Tower scheme.

The novices from day one are discouraged from outside loyalties for this same reason. Visitors are permitted to come to them, but there is no suggestion that any initiate is ever released for visitation before ending her training one way or another. Despite reliable contraception being available, they forbid the novices sexual relationships with men. Not with other novices who are part of the Tower, despite such relationships being an even more likely source of drama and distraction than a normal relationship, as the number of women in such a relationship is doubled (making potential for emotional drama and upheavals SQUARED). Better to have a sexual relationship with another initiate than a friendship with a male who is by definition an outsider, no matter how much worse such a relationship would be by all the rationalizations they give for forbidding boyfriends. It is part of the Tower pounding into their initiates the notion of "Tower good, outsiders bad."

We see from Faile's recitation of examples of Aes Sedai personal involvement in past events, & detect hints and implictions elsewhere, that one way or another, the Tower WOULD get compliance from its daughters, or else make them dearly regret their recalcitrance. Sisters undoubtedly know this and are made to understand before ever taking the test for the shawl what is expected of them. This, I think, is what leads them to forgo children, as they would not want to put themselves or a child they care for in that position. It does not have to be stated and can be every bit as unconscious as the motivation for the mass conception of children by the camp followers of the army in New Spring.