I might be completely off, but... I'm not so sure he knows she is cheating. He saw the other man in "Smiling Swine" and I think he began to wonder if she was cheating on him.
Since then, the question has been eating at him and finally he explodes. He guesses (perhaps because of his mother) that Ms. Leading is a prostitute and has been playing him, so he yells at her and treats her like a prostitute.
Then he stops himself and tells her to put her clothes back on. He feels ashamed for having treated her like that. This time, he takes a calmer approach and tells her that he doesn't know whether or not she is a prostitute and that he would probably have been paranoid no matter what.
He also realizes because of this display of emotion that he is much more in love with her than he thought, and thus more vulnerable to getting hurt. This thought is what leads him down the path of breaking up with her.
I might be completely off, but... I'm not so sure he knows she is cheating. He saw the other man in "Smiling Swine" and I think he began to wonder if she was cheating on him.
Since then, the question has been eating at him and finally he explodes. He guesses (perhaps because of his mother) that Ms. Leading is a prostitute and has been playing him, so he yells at her and treats her like a prostitute.
Then he stops himself and tells her to put her clothes back on. He feels ashamed for having treated her like that. This time, he takes a calmer approach and tells her that he doesn't know whether or not she is a prostitute and that he would probably have been paranoid no matter what.
He also realizes because of this display of emotion that he is much more in love with her than he thought, and thus more vulnerable to getting hurt. This thought is what leads him down the path of breaking up with her.