How do you determine "sexual fitness?" What does that mean anyways? Does it mean someone who has lots of sex? Then all you've said is a tautology. Do you mean someone who has pleasurable sex? Most people who have a lot of promiscuous sex find it loses its appeal, and they have to have more of it to get the same "buzz." Then, you have to consider how you measure pleasure. Are you measuring it by who has a lot of sex? I.e. someone who can get laid with a lot of people must give the other a lot of pleasure? Given my alternate analysis, these people may have a lot of sex because they've lost the real pleasure behind sex.

At any rate, such a criteria is much more subjective and open to a wider variety of interpretations than "morality." Whether you believe morality principles are based on objective reality or not, moral principles are codified in certain behavioral standards that are measurable, unlike something such as "sexual fitness."