They Constantly Talk About How Other People Cheat On Their Partners
You should be really concerned when your partner starts to spew this nonsense. This belief is even supported by some writers and psychologists. Their thesis is that cheating is an integral part of a relationship and that having an affair can be a positive outcome. Basically, cheating will lead you to have a longer lasting and happier relationship.
Maybe it works for some people but it certainly is not a healthy way to maintain your relationship. This rationale might even be used to justify why they are cheating at the moment.
This will certainly have deleterious effects. If your partner starts cheating on you and says that it is normal then you might as well cheat and it will become an endless cycle of cheating.