I'd been meaning to blog about a very interesting comment that Apollodorus left for Scott Carson on Examined Life. However, time has been short for writing long, thoughtful posts lately, and I see today Scott has put up an even more interesting response to the question himself.
Scott, of course, actually knows what he is doing on philosophical ground, whereas I just flounder about at times as in interested amateur.
The interesting question, for me, what what you can say about human nature and human teleology from a strictly naturalistic framework vs. what requires you to assume the existence of some supra-natural element in humanity, whether you call it the "mind", "soul" or "image of God".
It seems to me that there are teleological elements to the human person that can be discerned within a naturalistic/evolutionary framework. For instance, biologically speaking humans possess sex organs and the capacity for intercourse for the purpose of mating with the opposite sex and producing offspring. Biologically speaking, that is the only reason we have sex organs. Thus, biologically speaking, both homosexual intercourse and contraception are contrary to the biological purpose of those aspects of our bodies.
Similarly, it seems in every discussion of homosexuality someone on the pro side says, "The existence of hermaphrodites and trans-sexuals proves that gender categories are flexible if not meaningless. Thus, the important thing is orientation, not plumbing."
One might, I suppose, argue this from a psychological or theological point of view, but it's clear that one cannot make that case from a biological point of view. From an evolutionary point of view, an organism with non-functional sex characteristics (whether through an inability to mate and produce offspring or a refusal to do so) is not a correctly functioning organism.
However, what one cannot get from this naturalistic teleology is a moral imperative. While we can say with surety that homosexual marriage is contrary to our biological purpose, we cannot say at a purely naturalistic level that it is wrong to engage in homosexual relationships to the exclusion of heterosexual reproduction. Biology is incapable of assigning moral values, it simply states that an behavior, characteristic or individual organism "successful" or a "failure", or some gradation of more or less successful.
It does kind of seem to me like a lot of the things we traditionally say about "human nature" assume some sort of mind or soul which goes beyond a purely naturalistic concept of the human person. Indeed, that seems to me to be one of the strongest arguments that there is more to the human person than mere biology. We are, in part, mostly hairless primates with large brains and a strong mating instinct. But it seems to me that to assert that we are no more than that runs so clearly contrary to one's experience of being human as to be almost laughable -- though still just plausible enough to be terrifying.