I think beauty in most things has some objectivity, some subjectivity. Ever hear of the golden ratio? Symmetry? Most people find these things very appealing. If 99.9% of people do I would say those are objectiviely beautiful, desireable traits.
There are things we all find unattractive. Acne, morbid obesity, missing teeth. Sure, we can overlook these things, or they can be shadowed by great traits, but all-in-all we can agree that these things are not attractive.
Then there is the middle ground. I like curvy girls with full hips. Some people like thin as rail girls with small features. Someone may like chiseled guys where another girl might prefer a softer body type on a guy. Brown hair might be desired over blonde.
I think we can find the extremes and easily label them attractive or unattractive mostly objectively. It is the middle that we all argue about.
Yes, some people do have ugly kids. They think they are beautiful because they have to. Most people cannot look objectively at their offspring and say "Oh, I would change xyz if I could."