Donk: Interesting post. Here's a couple of thoughts:
First, I'll just observe that not everyone believes in the validity of evolutionary theory. But your post starts with the premise that we'll assume it's valid, so I'll leave anything more on that issue unsaid.
Second, assuming the general validity of evolution/ natural selection theory as a premise, the next step would be to state a theory of how natural selection plays out as an empirical matter. I'm no scientist, but it seems to me that a theory that does not explain what we actually see in the world must be of questionable validity. Taking your version of natural selection theory to its logical conclusion, there would be one ideal size for everything and we might expect all human beings to be about that same size (i.e., in overall body size, not just penis size). And that's obviously not the case--there is great variation. (On the topic of penis size in particular, it may not be that a 9 inch penis is a genetic advantage, but if it were a serious disadvantage, it's unlikely that the genes for it would still be in existence. On the other hand, maybe all of us with large penises are just mutants.)
Third, assuming, for the sake of argument, that the average-sized penis is perfectly suited for its purpose and that this explains why most penises are around that size, so what? Is there some moral victory in having an "evolutionarily perfect" penis size? As a topic for discussion, the post is interesting, but to the extent that it suggests there is something "wrong" with having a penis that varies from the median I don't really get it. (Also, I don't really agree with the post's premise that LPSG members generally perceive a "genetic advantage" to a large penis. And I personally don't believe that my penis size makes me in any way superior to anyone with a smaller penis.)