For the vast majority of recorded history people married someone from their village of 200 people or from a nearby village a few miles away. It doesn't matter how weird our mating rituals are when there's not much variability to begin with.
Modern urban cities provide access to a much more diverse gene pool which allows individuals to select across a much wider range of characteristics.
Sexual selection is invoked to explain all sorts of things that seem counter-intuitive at face value. The best know example is peacock feathers, which make males more susceptible to predators and are energetically costly. Thus, a male who has an elaborate display must be healthier and more vigorous than one that does not.
Modern urban cities provide access to a much more diverse gene pool which allows individuals to select across a much wider range of characteristics.