Course the fact that Lois could even have Superman's kid blows... literally... Niven's "Man of Steel, Women of Kleenex" theory.
Only half of it, that she would be unable to carry a superfetus to term. Apparently, the lack of yellow sunlight in the womb helps in that regard, as does the fact that the baby is half human.
But, it does nothing to dissuade us from the belief that Superman's shotgun-like ejaculation would completely annihilate Lois if they had sex. The only reason they were able to make love in Superman 2 is that Clark gave up his powers and was no stronger than a mortal human at the time.
And that's as good an explanation as any for why Superman left her after getting his powers back.
I don't even buy the "Woman of Kleenex" theory. At least it's not something I'd like to take seriously. I don't want to get too graphic here, but besides, maybe Kryptonian physiology is different from humans. I mean, what if Clark/Superman just ... oozes out ... or maybe it's similar to an Earth man's ejaculate (even if he wasn't at 100% full strength when he made love to Lois in S2).
On the other hand, I remember an episode of Lois and Clark
many years ago in which Lois asked Clark why his Superman costume didn't get ruined (at least not all the time) during a fight or an explosion, etc. Clark's response was that there was some kind of a "protective aura" that emanated from his body (e.g., generated by his skin a few centimeters thick) that protected his super costume. This would also explain how Lois could survive sexual intercourse with the Man of Steel. I don't know if that's considered canon, though.