A different way of summarizing telos that may be helpful when discussing this topic is that an object or agent fulfilling it's telos is supposed to be the most fully itself. So, for Aquinas and Aristotle, because the quality that most clearly sets off the category 'human' is "a reasoning animal" actions that interfere with that part of human identity are unnatural and interfere with telos.

When Charlton Heston was on the Planet of the Apes and he found that human beings were no longer differentiated by their reasoning powers (which were sub-par) but by their hairlessness, should he have devoted his life to keeping exceptionally well-shaved?

(this question brought to you by my continuing confusion with teleology)