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.
Much of what your sighting wasn't cannon yet, and given the internet didn't exist, nor did the kind of nerd culture we have today, it's entirely reasonable for a writer at the time to not be expected to have encyclopedic knowledge of Superman canon. Insofar as there even was true canon given it was the twilight of the Silver Age, when "bonkers" was the name of the game.
Moreover, that eassy is pretty famous for being such a candid, deep dive into such a silly topic but bringing up something most people at the time hadn't considered. This was 1969, published in a mens magazine, by a sci-fi writer, that basically took on meme status. It's pretty obviously not meant to be taken seriously.
Overly serious people on the internet are unable to contextualize a piece of speculative literature given the time and place in which it was written? On my Lemmy instance? Never...