True beauty = perfection
My interpretation of perfection may be quite different from yours, as may the things or people I consider to be perfect. But I think the reason we find things beautiful is because we consider the beautiful to be the most perfect example of that thing we have ever seen.
Yes, beauty may be based solely on appearance, on the amount of work it has taken to create something, or because of some indefinable quality that we just find fascinating.
I love fashion, and for me the most beautiful dresses were made by
Charles James. They're not practical in the slightest, I know I'll never own one, or, if I was lucky enough to, I'd never wear it. For me these dresses are beautiful because they are the most perfect examples of form that I have ever seen. They were immaculately constructed (which is part of their appeal), they were made of the most exquisite materials, and they are, quite simply, beautiful to look at. The fact that they are dresses is irrelevant.
Sunsets can be beautiful, animals can be beautiful, cars can be beautiful, as can people. To think of beauty solely in terms of physical / sexual attractiveness is a mistake.
True beauty can be appreciated by everyone, no matter what the beautiful thing is. I am not a fan of modern architecture, yet I can see an elegance in simple lines and clear open spaces. Although I don't find it beautiful myself, the fact that I can see something pleasing in it must mean that real beauty is something that cuts through personal prejudices.