If we assume that life on earth popped into existence at God's whim, there's no reason to expect the massive emphasis on sex that we observe in the natural world—especially if he's so persnickety about how humans perform the act. But sex lies at the very core of evolution, and its implications echo through every aspect of animal behavior. Males in most species have almost no investment in their offspring, so they're programmed for promiscuity. Creating offspring is a huge investment for females, so they're hard-wired for choosiness.
This tension between male and female behavior results in the phenomenon of sexual selection. Males are forced to compete for mating opportunities. Sometimes this manifests through direct battle: "the clashing antlers of deer, the stabbing horns of the stag beetle." In other cases the competition can even occur after sex. Coyne describes several clever and even devious strategies, including male millipedes that ride females for days to block the access of other males and damselflies that use spines on their penises to scoop out the semen of previous suitors. By religious standards, God's supposed creation is both ruthless and sexually depraved.
Sexual selection also provides an answer to one apparent problem for evolution: the males of some species possess traits called sexual dimorphisms, which in many cases are oddly cumbersome. For example, the feathers of the male peacocks are heavy and easily noticeable to predators, thus greatly reducing their apparent fitness. Why would they have evolved?
In effect, these traits tell females that the male is capable of survival in spite of these handicaps. In the case of peacocks, the longer and more numerous the vibrant feathers, the more fit the male is, and the more attractive he is to the female. In one experiment, scientists painstakingly clipped eyespots from some peacock tails and even glued them to other peacocks. The result: more eyespots meant dramatically more mating opportunities. Another experiment showed that "males with more eyespots produce young that not only grow faster but also survive better."
The rule holds for other species as well. Female house finches also prefer bright red males because their coloring indicates a healthy diet of pigment-filled seeds. Female gray tree frogs prefer males with longer calls—which, it turns out, produce tadpoles that grow faster and larger. Thus, the major elements comprising sexual selection have been confirmed through repeated experiment and observation.
The power of natural sexual selection as a theory is that it makes testable predictions. First, if one gender displays more flamboyant physical traits or behavior, that gender is the one that will be competing for the opportunity to mate. Second, we should observe that species that tend to be more monogomous (like penguins or geese) will show less sexual dimorphism, since sexual competition and selection play less of a role. Both of these predictions have been widely confirmed—and there's no reason for this to be the case unless evolution is true.