They want you to overlook their cheating
A spouse who cheats, even if it happened “just once” might ruin a relationship. Being unfaithful is probably one of the biggest heartbreaks one could ever cause, so no wonder some people never really get over it.
This kind of indiscretion can be defeated in time, with a lot of patience, and maybe even therapy. It requires 100% commitment from both partners, as they both need to make things work.
However, if the cheater seems extremely relaxed and constantly repeats that you should “allow it to pass” and to “get over it”, it can only lead to separation. Things require time, and when it comes to breaking trust, it’s even more important to be patient.