http://www.science20.com/challenging_na ... t_kill_him "Surprisingly, at the appropriate time of development, an embryo can implant itself into almost any living tissue that it happens to alight upon."
Male pregnancy was suggested to avert the problem of a kryptonian child developing within Lois Lane and killing her with its super-powerful fetus limbs. This was after it was established that Kal-El's semen are, to scale, just as powerful as he is, and upon release into the vaginal canal (if the hydraulic pressure from his immensely powerful ejaculation didn't rip her head off), the super-semen would actively search for any viable eggs in the vicinity. After the first egg was impregnated, its weak human based defenses would be no match for Kryptonian sperm, and it would either be filled by every other sperm that could fit inside of it, or they would change course and search for all other viable eggs in the city of metropolis. This would cause millions of microscopic perforations within the abdomen of Lois, ultimately resulting in her death due to infection. The sperm would grow more powerful in sunlight, have the ability to move at near light-speed, and be able to travel through any material as easily as they could travel through vaginal mucous and semen. This would result in millions of pregnancies in the city of metropolis, as the cells would only die after running out of biochemical energy, though it's suggested that Superman only eats out of habit and can actually survive on sunlight alone. So, basically, Superman would have to select a single sperm, gather an egg from Lois, and implant the egg into his own body to develop a child and safely propagate the Kryptonian race (because Supergirl is his first cousin). This would lead to much confusion amongst the people of the world watching a pregnant Superman save the day, and possibly some clever jabs from both his enemies and fellow heroes.
I suppose it can be, but fetuses attaching to tissues they aren't meant to attach to sounds gross and painful.
According to the article, it's also very dangerous due to the placenta merging with whatever structure it can gather blood from. This means that, even if the development of the fetus doesn't directly jeopardize the health of the host, the removal of the baby and the placenta will cause severe bleeding that is difficult to stop with today's medical practices, due to its need to be directly cut from whatever it's growing off of. I suppose, when medical technology advances enough, this would allow children to be born to couples where the woman doesn't have a functioning uterus (but still has ovaries) and no surrogate can be found.
It sounds like superman should just impregnate his cousin. The risk of expressing genetic degradation is quite small, and certainly less of a problem than the scenario described above, ie, supersperm hunting down every ovum in metropolis. (Depending on the recessive genes he shares with Supergirl, of course. Possibly a severely retarded kryptonian would be quite a problem.) One has to assume that he spent puberty concientiously ejaculating into the sun in order to avert just that sort of disaster.
He would definitely have to be outside while punching his way into heaven. After the first hole blown through the ceiling, it would be hard enough to explain to his parents. As far as I can tell, sperm responds to chemical cues and tends to speed up dramatically when it catches the right signal. If he were to bust into the sun, it would dramatically decrease the potential boom in population. I learned that sperm really likes the smell of L-tryptophan...at least earthly sperm does. I would hope that the chemical signature was blocked by the composition of his semen to prevent the propagation of millions microscopic wormholes and sonic booms in the sperm's desperate attempt to rush a fertile egg. He's got the option of Supergirl and Powergirl, they're both his cousins, though Powergirl is from a parallel universe. It's been shown that genetic degredation between cousins is only slightly more serious than two essentially unrelated people. However, there's a very small pool of gene variety, so that would only be safe for the generation created by Superman and his cousins. Any further than that and weird stuff happens, assuming Kryptonian DNA acts anything like good ol' human DNA. Really, the only person who could possibly impregnate either of those Kryptonian women is Superman. For one, the muscles in their vaginas would either completely prevent penetration or, if the guy was lucky (and this is the worst kind of lucky ever), his manhood would be destroyed. Crushed beyond all recognition. If he survived unscathed and somehow his sperm made it (by complete failure of the Kryptonian immune system), it would have no way to get through the cellular wall of the egg. Now, if some method of fertilization was devised (possibly by weakening the eggs with kryptonite), in vitro could work. Gold kryptonite is an option, as it removes a Kryptonian's powers permanently while leaving their genes intact. However, that means leaving the current generation essentially powerless. But, the kryptonite could be used on either the eggs or sperm, which leaves it to be seen if the kryptonian genes are recessive or dominant. If they're recessive, normal human baby. Dominant means the need for a super-uterus. Imagine a fetus with X-ray vision; enough of that could sterilize a human mother. Now imagine heat-vision. Now, as I said before, a super fetus punching its way out into the open air. It's not a pretty sight.
Or he just impregnates Lois on a planet with a red sun, wait till the child is born and they return to earth. Short of that he wears blue kryptonite on his person while doing his thing as does Lois. It should keep the baby human till delivered.
Science is just a bunch of fairy stories invented by the government to distract us from crop circles.
I saw in a newer comic that Superman wears a red sun ring to allow for a more human-like physicality with Lois while he's Clark Kent. But, you're right, he could easily go to a planet with a red sun and do the deed. Blue kryptonite works that way in Smallville (which I stopped watching after the 1st season), but since my only reference was comic books I didn't consider it; in the comics I've read, blue kryptonite hurts Bizarros and makes Kryptonians stronger. It may have been changed to allow the "reality" of the stories in Smallville and the comics to mesh better, but I don't really know, because I haven't seen a new storyline in the comics with the blue variety in it, mostly because I don't buy comics anymore and only end up reading them while in the appropriate store. If that's the case, blue kryptonite should allow Superman to do his thing safely.