1. Unless there are kids involved, in which case the fallout extends beyond the relationship and has wider second-and-third-order effects on society. Aren't something like 40% of children in single-parent households in the US? And aren't there significant statistics showing that especially fatherless sons are over-represented in juvenile crime, etc...? Those situations might arise less if the population didn't have such a blasé attitude to starting and ending romantic entanglements.
2. Studies have shown that increasing numbers of sex partners has a permanent deleterious effect on a person's ability to pair-bond, with more serious psychological impacts on women than men. (sorry, don't have a citation handy) So it is not a purely "good and healthy" situation for people to be regularly growing tired of their partners.
I'm 38, essentially polygynous, with my primary female one I selected almost 10 years ago for character traits that I assessed as optimal for raising children and managing a household. So far, so good on both fronts. We have some SLIGHT overlap in our movie tastes such that we can usually find something mutually satisfying (mostly lighthearted action movies with a touch of romance). If I want to watch something extra graphic (13 Assassins, original Robocop, etc...) I'll either watch it alone or call one of the side-chicks that is comfortable with that stuff.