This seems so obvious that I can't believe it would actually be necessary for someone like me to blog about it. Evolution is a scientific theory that requires that creatures are preeminently designed to carry on their species, genes, etc. Biologists recognize that deviations from this basic pattern require special explanation if the theory is to be maintained. Thus, they explain mothers protecting their offspring from the standpoint of genetic survival: the mother is likely to have fewer additional offspring than the progeny from that point. Also, the stripes on a zebra can only be explained on the level of the herd, not on the individual. These are special cases. Ordinarily, evolution is explained on a one-to-one basis. This monkey is good at climbing trees because that keeps it safe from lions and enables it to access more bananas.
It translates into the human order, say the scientists. Here they distinguish between biological and cultural evolution. I won't go into that too much. On the human level, we can see that our huge brains have evolved to enable us to outsmart lions, tigers and various microbes. Our hands can hold weapons and tools. Our legs can help us to chase down rabbits and deer. Our genitals are specially designed to aid in copulation. Women like men's broad shoulders; men like women's supple breasts, not for purely aethetical reasons, but because broad shoulders tend to mean protection, supple breasts tend to mean adequately-nourished offspring.
Men being attracted to men makes no sense on the biological level. Psychologists never attempt to explain it that way. They treat it as the result of a psychological trauma, a malformation, because it does not make sense according to the biology of evolution.
I have heard homosexualists explain it on the side of population control, that it is sometimes advantageous to a species to limit its growth. But they offer no explanation for how this works. Actually, this is absurd, biologically speaking. It argues for a super-conscience in the species as a whole. Absurd.
Rather, humans can deviate from the strict one-to-one account of evolutionary biology because they can make decision irrespective of the survival of their genes. We do it all the time: we smoke, we eat junk food, knowing that these things are bad for us; we neuter ourselves by means of surgical intervention. This is not an adaptation; it is a choice.
Homosexuality is not an adaptation that increases our biological fitness; it is a choice that decreases it.
Either speciation is explained by evolution or it is not. Either human sexuality is explained by the basic law of propagation and survival or it is not. If homosexuality is 'natural' evolution is false, because in it we find the appearance of a trait that cannot be explained by evolutionary theory.
Obviously, it fits into the category of social adaptation,or of sociological not of biological phenomena.
That is the easier side of the argument.
But there are two sides to this disjunctive syllogism. If both cannot be true, can both be false?
Logically, if it follows that
(If A then not B) and (If B then not A) are sound
it cannot follow that (not A and not B)
because evolution suggests that all natural characteristics are the results of some (historical) fitness. Homosexualism can never be a form of fitness.
So (not A and not B) amounts to (not (everything is a result of fitness) and (there is a thing that is not the result of fitness).
So, as the first part of my argument was an attack on homosexualism (i.e. that homosexuality is natural), this second part is an attack on those who deny evolution. Since, to deny evolution is to bolster the hand of homosexualists.
I have said that it is not possible for both A and B to be true at the same time. I have also said that it is not possible for both of them to be false at the same time.
Alas, I have overstreatched things. I should have understood B in the broader sense as some (any) violation of the rule that all traits of creatures is a result of historical fitness! Homosexualism is an example of a -A.
So, to deny A is to require some B. Homosexualism, then, becomes logically possible, although not necessary, because even if H is a B, not all Bs are Hs.
So, to those who would deny evolution, be careful, you are undermining the strongest argument against the 'naturalness' of homosexuality.