I think it's stated clearly that Thrakhath is an exceptional case when it comes to being able to bear young. As hard as he tries (what a job), he can't seem to get healthy children from any of his harem. Also, the fact that his offspring is a sickly daughter is probably somewhat akin to Henry VIII's problems with heirs--the presence of an X or Y chromosome in sperm determines the sex of a child (so it was Henry's fault that he couldn't get a son, and chopping off his wives' heads was pointless).
Now, obviously we don't know if Kilrathi had a sexual reproduction even remotely resembling our own, but I think what Forstchen was trying to imply holds true: it was Thrakhath's fault that he couldn't bear robust children. It's also hinted that the imperial blood line had grown weak (although whether or not this was just on Kilrathi superstition, or whether it was based in genetic fact, is an open question).
I personally got the impression that Kilrathi bore litters, but I'll have to go back through the source material and see if there's any justification for that. After all, the Kilrathi presumably have large families, as we know that Thrakhath had a lot of relatives to take his place if he ever failed the emperor.