They Become Angry When You Confront Them
You’ve been meaning to confront them because you feel too suspicious of their behavior. When you finally do, they don’t know how to defend themselves in any other way and become irate with you.
They do that by saying that you don’t know what you’re talking about or they might even blame you for cheating on them. This, of course, is just a scheme to try and make you guilty of the crime that they committed. If you insist on them admitting their guilt, it can get worse and worse and you can end up fighting with each other.
Clearly, at this point, your partner is cheating because there is no other reasonable explanation for the behavior that they are exhibiting. Some might never admit that and they are those that will make you the antagonist in their version of the story.