I don't remember the exact words, but Clark definitely told Lois that "Dr. Klein said that Superman's [? something?] and an earth woman's are incompatible for reproduction." Whatever it was, the implication was clear that it had to do with Clark being an alien.
On the other hand, as has been pointed out, Dr. Klein was wrong. So, perhaps they didn't have an infertility problem at all. (That is, many writers assume they had a problem, but it could be solved. But if Dr. Klein was wrong in his assumptions, maybe there was no infertility problem, period. They had only been married a few months at most. I think most doctors let a couple try for a year before deciding there might be a problem.)
Bottom line - you can pretty much write it however you want.
- Vicki
Edited: Come to think of it, Faster Than a Speeding Vixon was the first show in which they "didn't take precautions". Then, later in the same show, they do, in fact, take precautions. So I don't know that they *ever* actually tried to get pregnant. I think they just asked Doc Klein, and then accepted his word for it.
