Most people seem to have a "type" to whom they're physically attracted. And it is often said that opposites attract. It seems that quite often, a person's "type" will be someone with whose genes they might combine with their own to produce good-looking offspring.
Therefore, people will often desire partners who will offset their own aesthetic flaws. Someone with a weak chin will be attracted to people with stronger chins, someone with a bulbous nose will be attracted to people with sharper noses, skinny will be attracted to strong, etc.
It's quite possible for two opposites, neither of whom is particularly good-looking, to produce attractive offspring. You see that all the time.
It make evolutionary sense that such an instinct -- attraction to opposites -- would have been bred into us. Genetic fitness is not purely a matter of having a lot of offspring yourself, it's a matter of having enough offspring who have their own offspring.