Why is the wife always prettier than the mistress?
Maybe it was like a charitable deal for him, or part of the compensation ‘package’ she negotiated when hired.
You have to look below the neck. Girlfriend is sleek and aerodynamic and wife is a wide-body. Plus, girlfriend is probably ok with in-flight refueling, whereas wifey says no to the blow.
It's not a question of who's prettier (although I think the mistress is slightly prettier). The mistress, by definition, is a woman who makes an effort to keep her guy sexually satisfied. If the wife has lost interest, then her looks are irrelevant.