Can it really be called infidelity or cheating when they were meant to be together from the start? Notice that Erwin married his wife because she took a liking to him and is the daughter of one of the investors or someone rich in the company. So, Erwin and Levi just return to how they were before. Note: I personally hate cheating and the likes, but considering they always were meant to be, it doesn't feel like they were cheating.
Attack on Titan dj - Others' Husbands