I think futas were originally nerfed in fertility because they got both dicks and tits, which both could increase fertility. They could both decrease it a bit too, but the potential increase was much higher. So to counteract this and add a bit of a penalty to them they got the fertility malus. But with the size traits no longer having any effect, futanari are very often going to simply be completely infertile.
Original URL: https://www.loverslab.com/topic/108569-ck2-dark-world-fantasy/?do=findComment&comment=2537031