This is a redated post.
When people say "the purpose of X is Y," what do they mean? Well, if what we are talking about was created by a human being, then we are talking about the intended purpose for the object. The purpose of glasses is to help you see something, the purpose of a baseball is to be used the game of baseball, the purpose of a car is to impress girls, etc. But what if the object is a natural object. The purpose of your eye is to see, but what does that mean. If you believe that they eye was intelligently designed, then we have an intended purpose. But what if you don't believe that natural objects were intelligently designed. Well, there is Darwinian function. Something exists because its performing a certain function allowed the thing to be selected for by evolution. The purpose of your eye is to see because it is the seeing capacity of the eye that permitted critters with eyes to survive.
Aristotle seems to have a concept of purpose that is neither an intended purpose nor a Darwinian purpose. It is a natural tendency of something to reach a certain goal. The metaphor here is that of the acorn and the oak tree. The natural progression of the acorn is toward an oak tree, and not toward a tomato plant. But although Aristotle believes in an unmoved mover, the unmoved mover does not intend for the acorn to become an oak, because the UMM is pure thought thinking itself and it is not even aware of the acorn.
Is this concept of natural purpose plausible? Sometimes people argue against homosexuality in virtue of the fact that it conflcts with the procreative natural purpose of sexuality. If "purpose" is intended purpose, then we are going to need some information from the one who has a purpose for us in order to know if that claim is correct. That would, of course, make the argument a religious argument. The fact that homosexuality is a Darwinian liability isn't much of an argument. After all, we have no moral mandate to reproduce as much as possible. So maybe what people have in mind is an Aristotelian sense. But is Aristotelian purpose plausible in the absence of intended purpose? I don't think so.