As I'm grading several papers on Aristotle's function argument, it occurs to me that we take the basics of his argument very seriously in a variety of contexts, and always assume the basic Greek worldview. It was basic to the Greek way of thinking that every thing, every species, every action has a unique function. Aristotle and others then use knowledge of that unique function to determine what a good instance of that thing may be. Many people still use the basics of that view when it comes to sex, and this has profound implications for the ethics of sex. But the assumption seems flat out wrong, and hence the ethics based on the function argument seems poor at best.
Aristotle argued that the word good is always contextual, getting its meaning from the noun it modified, and the noun gets its meaning by its particular function. So, a pianist is someone that plays the piano, and a good pianist is one that plays the piano well. When you find the unique function of an object, you can then understand what a "good" object maybe, be it a piano player, or a car.
He argued that the function of man is not mere nutrition and growth, because these attributes are shared with plants, and so are not man's unique function. He argued that sense perception and movement are not man's function, because these are shared with animals. Man's unique function is the use of reason, hence the function of man is to reason, and a good man reason's well, both in practical life as well as in the contemplative life.
The immediate effect of the argument is to place an emphasis on reason, and on the unique characteristics of man, separating him from being an animal. Hence those features of human existence that we share with animals are downgraded, and acting like animals is a bad thing. And if its a bad thing, then anything which takes us away from our rational, human nature is degrading and very bad.
If this argument is not right, then it is very easy to see why some philosophers (like Kant) holds that sex is inherently degrading, reducing us to animals, and hence morally reprehensible. Sex might be necessary to keep the species going, but not good in itself, not to be valued as anything except useful for procreation. (Of course it follows that if sex is valuable for its unique function, and the only function it has that is truly unique is procreation, that good sex is reproductive sex. You may think you have had good sex before, but if it did not produce offspring, you are wrong. And you might have thought that sex that cause a child was not all that, but again, you would be wrong. The best sex, according to this argument, is one where a child is conceived.)
We might grant that procreation is the only unique function sex has, but it is obvious sex has many other functions that are not unique. You might think, for example, that it brings couples closer. Everyone must grant other actions can do that as well, so it is not the unique function, but few can deny that it can also have that effect as well.
Here, then, is the question: Why would the obvious uniqueness of one function make the others irrelevant, or even elevate the unique function? Why is value tied to the unique function? Why cannot we attach meaning and hence value to any purpose we give to any act or person? Then, no longer accepting to split between animal and human, as we no longer accept the function argument, we need no longer look at our animalistic nature with horror and dread. Deny that the only function which counts is the unique function, and off we go with a very different conception of value and ethics. So what justifies the assumption either that function is unique or that only the unique function is the one that counts?