I don’t view “we’ve gotten bad at sex” as evil, though it can be a symptom of “falling out of love.” It just depends on the media in question and the story. Plus I can enjoy something even if I don’t agree with the protagonist’s actions.

Edit: When I say “we’ve gotten bad at sex” not being evil I mean on part of the person being cheated on not the cheater. Being bad at sex doesn’t make you evil and “deserve” to be cheated on.