In Superman II (1980), and I am guessing in some other sources, Superman relinquished his powers prior to engaging in sexual intercourse with Lois Lane. Why though? Was it because he couldn't have had sex with a human female otherwise? Or because a fetus he had fathered would have been dangerous for a human female to carry? Or is it simply genetic incompatibility?
I would argue that Superman does many things at the same speed as a human does -- it is, for example, not dangerous to be near him when he breathes. His hair, while not cuttable by normal tools, still grows at the rate that human hair does. I think it is implied that his sperm cells would be dangerously super-powered but if the moved at normal speed, like his hair growth, breathing, blood flow, would they be dangerous to either Lois herself or her eggs?