It's alll about aesthetics instead of what you've been conditioned to think is hot. Instead of looking for the usual things you look for, you see certain individual features that are nice-looking. Different colored skin, ears in interesting shapes, eyes that are interesting colors, muscles or curves that just...go a certain way that's pleasing to the eye.

Instead of thinking just "hey, that person has completely symmetrical features and looks like models on TV" you notice things more like you would in a painting.

Think about it, as a species, we can point to a painting and go "hey those colored shapes are beautiful" when the subject of the painting isn't even human. You kinda tap into that when thinking about whether an alien's hot.