Why did Superman relinquish his powers before pursuing a sexual relationship with Lois?

Question

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?

Answer 1

This was the subject of a famous humorous essay by the science fiction author Larry Niven: "Man of Steel, Woman of Kleenex.". According to Wikipedia, "The issues discussed include Superman's loss of physical control during intercourse, the presumed "super powers" of Superman's sperm cells, genetic incompatibility between humans and Kryptonians, and the dangers to the woman during gestation." Of the these, the last seems completely insuperable as a barrier to Superman fathering a child with a normal human woman.

Though exposed to gold kryptonite, the sperm still carries Kryptonian genes. If these are recessive, then LL carries a developing human fetus. There will be no more Supermen; but at least we need not worry about the mother's health.

But if some or all of the Kryptonian genes are dominant...

Can the infant use his x-ray vision before birth? After all, with such a power he can probably see through his own closed eyelids. That would leave LL sterile. If the kid starts using heat vision, things get even worse.

But when he starts to kick, it's all over. He will kick his way out into open air, killing himself and his mother.

Answer 2

In regard to Superman II specifically, there's no official explanation that I know of, so we can only speculate as to what the actual reason was.

Genetic compatibility for reproduction hasn't been an issue in some of the comicbook continuities though. For example, in the current Prime Earth continuity, Superman and Lois have a teenaged son named Jonathan Samuel Kent.

_{Convergence: Superman #2 (July, 2015)}

And in a non-canon Pre-Crisis story, titled "Whatever Happened to the Man of Tomorrow?", Lois and a retired Superman were shown to have an infant son, also named Jonathan. Superman relinquished his powers by exposing himself to Gold Kryptonite years prior to this, but the son was shown to be capable of crushing a lump of coal into a diamond, which would suggest that Superman still had Kryptonian genes, despite having lost his powers.

_{Action Comics Vol. 1 #583 (September, 1986)}

Superman and Lois also managed to conceive a child in an alternate future story branching off from the then-current Post-Crisis continuity. However, in that instance, Lois died as a result of internal injuries sustained from the super-strong fetus kicking within her womb.

_{Adventures of Superman Annual Vol. 1 #3 (October, 1991)}