I know that Morgoth couldn't produce offspring anyway, but I just thought I'd add an idea.
Morgoth would most likely
never have wanted a child. The theme of son-overpowering-powerful-God is recurrent in mythology - look at old Zeusy. A child to him would be a threat. So if he didn't know he was, er, sterile, I don't think he'd have wanted to risk conception. And they didn't have contraception in those days.
Quite odd of Old Tollers to make him sterile though. I can name a few evil people who managed kids...