The story that probably started with a misunderstanding.
If it turned out that the ex never cheated in the first place, that means the MC had something now on him.
Obviously, he did not trust his ex that much and just dismissed things as he saw it, not even bothering to ask for an explanation. Some might say that subconsciously, MC has an impression that his ex really has the tendency to cheat on him, sooner or later.
But IMO, even if they manage to clear up the misunderstanding, they should not get back together. It no longer is going to be the same anymore between them. It's like they will still break up sooner or later even without that incident. MC really probably more suitable to other girls instead.