That line was weird to me too. It looks like Saturns saying gorosei can’t reproduce.
I don't have the japanese raws on me, so I just translated the line straight from the arab scans we already have
"يتكاثرون" as in to "increase in number", or "reproduce the population"
also the Goroseis themselves unlike the other Celestial Dragons are never seen to partake in any worldly matters like sex and such. They're always seen sitting in their court room throughout the entire series.
Even during the Ohara flashback (20+ years ago) and their appearance has barely changed. It just goes to hint they might not be fully human.