I suggest that the physical properties of beauty are symmetry and complexity, and that beauty is also created by associations that connect with our emotions and what we desire, care about, or admire. Most beauty is made of both these physical and these subjective causes. The plumbing systems' being called beautiful, is primarily because they provoke admiration of the intelligence and work that made them, perhaps some because of the simple complexity and vastness of shape they present to the eye. People are more beautiful when their features are more symmetrical, but more so when their face shows feelings that appeal to us--if they appear kind, capable, pleasant, or intriguing.

Beauty indeed exists--the disagreement we find over what is beautiful is in part because the world is so complex that there are different aspects to almost anything you may look at, and different sides of things will be noticed by different people.

In response to Apeman:
symmetricality and complexity are not opposites. Consider faces; they are symmetrical and complex, and check out this wikipedia article: http://en.wikipedia.org/wiki/Facial_symmetry Symmetry is part of beauty, but complexity is probably a greater part; a box is not really beautiful, since it's too simple.