More likely, the more attractive partner has other issues that reduce their sexual market value:
Despite what many people think, opposites very rarely attract.
In fact, decades of research has shown that attraction is most likely to be sparked when two people perceive themselves as being very similar to each other.
But similar how? It could be similarity in terms of sociodemographics – most relationships are formed between people who are similar in terms of age, social class, occupational background, and so on.
But more important than sociodemographics is similarity of values – everything from musical tastes to political orientation.
We're all motivated to think that our views of the world are right and when someone disagrees with us, we feel uncomfortable in their presence.
But when someone agrees with us, they validate our worldviews and as result we want continuing contact with that person.
I've noticed that, in general, most people match up relatively well in terms of how attractive both spouses are. Sure, sometimes aging differently throws things out of whack, or there are other non-physical attractions involved, but one seldom sees true opposites together, as opposed to simple complementary pairings such as an introvert with an extrovert.