Ejaculation of semen is entirely involuntary in the human male, and in all other forms of terrestrial life. It would be unreasonable to assume otherwise for a kryptonian
Why exactly would the author jump to this conclusion? First it’s noted that Superman has precise muscle control. He can and has altered his voice using such a technique. Second he says it right there, he’s not terrestrial. Why would he have the same involuntary spasms from orgasm as an earthling? Even if he didn’t have his super muscular control there’s no cannon on how goofy a face kryptonians make when they nut.
It goes on to say the sperm cells would be super powered, but they probably need a steady dose of yellow sunlight before they’re ripping holes in ovaries.
This isn’t even getting into the theory that Superman isn’t super strong, he’s just a very powerful short range telekinetic psychic. Save that one for another time.
He was married and having kids before his theories had fully taken shape.
But he did know, or at least suspected, he'd made a mistake after the death of his first daughter. But he also had healthy children too, so he couldn't say anything unequivocally.
He ended up being a case study for his own theory, and he was well aware of it.
i was not familiar with their actual lineage, but its plausible that if supergirls 'dad' had married a single mom (supergirls mom), but wasnt responsible for that kids conception they wouldnt technically share dna.