@MarioRedfield: The Off-Topic board is for threads that aren't related to anime or manga at all; subjects like food or politics for example.
It's kind of the only response to give, because it's literally decided by the writers. If you want to take the comparative angle and say "well Goku is humanoid and reproduces like a human, Superman is the same way, so why can't he have kids?" then of course I'm with you on that, but that's obvious. Could Superman have kids with Lois? Sure, like I said, it's happened in some Superman adaptations when the writers want it to.
I suppose if you're looking for a "technical" reason, since there's no physical one, then I would have to say Kryptonian and Human DNA simply aren't compatible, while Saiyan and Human DNA are.
Again though, that's a totally obvious answer. I'm not sure what you're looking for since there's really nothing else to it, you see what I mean?