This film is reportedly a sequel to Richard Lester's "Superman II." In that film, you will recall, Clark lost his powers for a period of time and bedded Lois. I would assume the child stems from that incident.
LOL. I recall reading that many years ago. Larry Niven is a kookie dude. And I dig that about him.
That was a hilarious short story. I'm an incurable Niven fan.