Sure, there are some characteristics of things (music, faces, etc.) that are objective. I believe the assessment of beauty occurs in the human brain, and processes occurring in the brain I view as subjective because they are specific to individuals.Why? I don’t find the subject that simple.
Nature does not act to guarantee procreation. The laws of nature act on all objects in the universe, and one outcome is that there are some objects that reproduce. Another outcome is that some large objects collide with other large objects causing the extinction of some species. I would not say nature acts to guarantee the extinction of species.I find that to be a distinction without a difference. Your final sentence conflicts with the sentence that precedes it. In the end, nature is using the parameters that humans interpret as ‘beauty’ to guarantee the procreation of the species.
Maybe evolution is not the best topic to discuss when viewing the objective/subjective nature of music on a classical music forum. At any rate, I think it's not productive for me to continue.