Well if I understand you correctly. Your debate is why are we alive as individuals, whats our purpose. Those are two different things. Your opening line suggests one possible scenerio as to how a person is brought into this world. A screw up child birth. That doesn't represent purpose just the reason for existence. Those are two different things. Purpose is the reason you live for, not the reason you are alive.

"Your purpose? I'd say: to have sex, and thereby continue the species and add to the number of people who may ask these excruciatingly clichéd questions."

Is this really all you can fathom? this is what I have beleived myself since I started studying science seriously almost a decade ago, but recently I've grown dissatisfied with it, it is too nihilistic. If you would please indulge me, perhaps we can put this idea under the microscope and perhaps via a process of mental mitosis we can engender it to give birth to some progeny. I mean, assuming selfish gene theory to be correct - then selection takes place at the level of the individual genes only, but genes have no motives, they are simply replicators, but genes don't drive evolution, the environment does, they merely act as a base from which a species can evolve. However, if you think life is only about preserving your genes, and evolving, this creates a contradiciton (in my mind at least), as human living consists of far more than simply 'making a living' in order copulate with a female, reproduce, and thereby spread your genes. I mean, can you really say this is all that is at play here?