If that's the case, then how did sex evolve in the first place?
That's the paradox, see?
That must be where the "man and female created He them" from Genesis comes in.
Carolyn
Sex is the physical and behavioral equivalent of consciousness; it is the impetus behind everything we do: propagation/marriage - procreation, religion/mathematics - go forth and multiply, dealing with others/effectively communicating - "they're all F**ked up", "what a screwed-up mess", "what the f**k!", "don't f**k with me.", "We're screwed.", PETA - animal husbandry and sheepherding, entrepreneurship - prostitution, the porn industry,.... Then again, maybe not.
No paradox, actually.
Sexual reproduction can easily evolve in a number of ways, including the following sequence:
1. Start with single-cell organisms which reproduce by splitting, like most bacteria.
2. At some point one of the organisms stumbles upon a way to insert its DNA into other similar organisms, thus reproducing by "proxy" like a cuckoo bird which lays its eggs in other birds' nests. The evolutionary advantage to this should be obvious. And many microorganisms do this even today.
3. Eventually there's a whole host of its (direct and indirect) offspring around, all forcibly "impregnating" each other and cross-sharing DNA.
4. Since this is a valuable trait-sharing method, over time the species refines the process so that each individual is specialized at both "donating" DNA and being "receptive" to DNA from others. At this point there is true sexual reproduction taking place, even though there are no separate "males" and "females" -- all individuals are effectively hermaphrodites.
5. Later (and this can take place at either the single-cell stage, or a zillion generations later after the species has become hermaphroditic multi-celled creatures, like say earthworms), a mutant arises which has lost the ability to donate DNA, and becomes the first exclusively "female" organism in a population of hermaphrodites. No problem, it can still reproduce by receiving DNA from the hermaphrodites, even if it can't donate any DNA to others. It, and its female offspring, will still prosper and interbreed with the rest of the "male+female" population. Or perhaps a "male" arises first by losing the "receptive to DNA" structure -- same argument either way, it can still reproduce by donating DNA to others.
6. If females arise first, there's evolutionary pressure for at least some of the remaining hermaphrodites to specialize in impregnating the female form of the species. Over time they're likely to specialize so much that they become "males only". Or vice versa for the rise of a female type if males first arose in the population. Eventually hermaphroditic forms can fade away entirely, leaving nothing but "pure males" and "pure females" as the two dimorphic forms of the same species.