I like this idea. When we have to stretch too far to look for an explanation of a trait based only on that trait's effect on differential reproduction, it may be because there is no such explanation. Plenty of traits are the result of side effects that did not affect reproduction, and others may be cultural.

This idea has just what we need: it fits the experience, doesn't seem to affect reproduction, and is a side effect of sexually selected traits. When you add in a cultural component that may amplify or suppress this feeling of sympathy, you have what looks like a good explanation with no "just so"s necessary.