[QUOTE=Excalibre]
According to Wikipedia, the spermatozoon penetrates an outer layer, then releases enzymes that dissolve a spot in the next layer, after which a chemical reaction renders the ovum impenetrable to other sperm. So it seems that the chemicals released by the sperm are important. If Supergirl’s eggs are as impenetrable as her bulletproof skin, fertilization by mere human sperm ain’t gonna do anything.
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The big fallacy of these arguments is the assumption that all of Superman/Supergirl’s systems work in a matter analogous to his gross physical strength and invulnerablility. That need not be the case.
For example, Superman is invulnerable to radiation (excluding kryptonite, of course). Yet he can see. In fact, he can see much much better than a normal human. Light can pass through his cornea and lens, and can react with his retina, but it cannot burn or blind him. In the same way, I’d expect that a sperm would be able to permeate Supergirl’s ova (in fact, I’d wager that her ova would remain viable much longer than a normal female’s, and she’d likely be immune to chemical birth control, so wear a rubber, guys).
What I’m trying to say is that while empowered Kryptonians are resistant to injury, it doesn’t follow that they’re resistant to non-injurious manipulation. Supergirl’s vagina would be elastic enough to accomidate a penis, but wouldn’t stretch beyond that, to the point it was hurting her (so if Comet was hung like a horse, he may be out of luck). The hymen would remain a difficulty. If Supergirl wished to enjoy vaginal sex, she’s have to employ magic (a certain Mr. Batson would be happy to oblige), have it cut ahead of time by a gynecologist using a kryptonite scalpel, or simply another person of like strength, such as a randy Daxamite.
The part of the argument that remains convincing is that involuntary muscular contractions (vaginal contractions, ejaculation, labor, etc.) would have analogous strength to a Kryptonian’s concious muscular movements. This is given creedence by several incidence in which a super-sneeze erupts with hurricane force and the like. However, that was pre-crisis.
Post-crisis, Kryptonian powers have been explained as largely psionic in origin. Strength and flight becomes an extremely low-range form of telekinesis, telescopic vision a form of remote viewing, etc. It does not necessarily follow that a kryptonian would endow his or her autonomous systems with the strength. If they did, through practicing biofeedback, they could learn not to. Recently, Superman lost his powers for an entire year, and he suggests that he unconciously willed the powers not to return for that period because he enjoyed being human. He could therefore, with training, learn to turn some or all of his powers off.
Or else they could just do it under a red sun lamp. That’s the easiest solution.
