TomLikesGuitar said:
What is the biological purpose of humanity?
To procreate.
Any deviation from this is... unintended, to say the least.
...All of which sounds like you have read more science than most Americans. However, evolutionary biology shows it to be ALMOST correct.
See, the biological purpose of humanity is not to procreate. Procreation is the method used by human organisms to accomplish the actual point behind life: gene expression. Evolution favors any trait, process, or behavior that enhances fitness of a local gene population.
It is commonly misunderstood (but partially accurate) to believe this statement to mean that any trait that gets a guy/gal to live longer and laid more easily (taller, smarter, metabolize better) will be favored while those that make it less likely for an individual to have more successful reproduction (less sperm, bad at running from tigers, homosexual) will be suppressed.
Again, evolution acts on gene pools, not organisms. If a trait is more beneficial to the population than it is to the individual, it will be enhanced over the whatever makes the individual fittest to survive. For example, many primate males (including humans) will invest more resources in their sister's child than their own, even to the point of killing their own children. The reason gets complicated, but a male is more sure that his genes are actually at least partially represented in his sister's child (1/4 definitely of his genes) than his mate's child (who often may be deceiving the male to gain more resources for her own genes).
Homosexual behavior is used in many species, and arguably in our own, to foster bonding among populations and enhance the fitness over the general population. Many behaviors in humans fall under the category of improving the gene pool's fitness over their own. What about adoptive (presumably heterosexual) parents? They aren't procreating. But the underlying behavior to care for the population remains.
I like that people have an interest in evolution, but sometimes the subtleties are not taught as well in a sophomore biology class for someone discern between the "sounds correct" and the "is correct". I look forward to having children soon and I will do my best to see that the seemingly reasonable fallacy presented by TomLikesGuitar dies with my generation.