Okay, so you've necromance'd a thread; anything to add to the content itself, planetstella?
Looks like the replies break down as
1) because eugenics
2) because Yeerk society has a 1984 feel to it, with sex being a controlled commodity rendered as unenjoyable as possible so that it doesn't become an expression of individualistic goals and enjoyment
3) because if a Yeerk has human kids, they might love those kids too much to surrender them for infestation
4) because pregnancy creates a hormonally compromised situation that might make a host harder to control, or a Yeerk less effective at their job
5) because allowing Yeerks to bond closely with each other would diminish the competitiveness of Yeerk culture and encourage divisive personal loyalties
6) because pregnancy renders the uterus-having host a lot less physically capable of work, for the last few months before the birth, and for a month or two after birth
and on a meta-level relating to the authors' own ethical insights,
7) because it would be really creepy and demographic-inappropriate to write any situation where an infested human has sex, since in a majority of cases, the consent issues here are huge and obvious, and the authors preferred to draw the line at making hosts kill other human beings
Have we missed any?