What happens is that the chick cheats on him, he finds out and leaves. He gets support by that chicks sister. The chick that cheated on him realizes that she really loved him and only him all along and tries to 'make up' for it. She quits her job, gets a new one (she cheated with someone from work) and moves elsewhere. She also finds out that she is pregnant (and that is certainly is not the mc's child) so she secretly aborts it.

Finally she gets him to move in with her again but is dismaid because he clearly still can't really stand her and is forcing himself to be with her again.

The last chapter then reveals that he and her sister were planning to make her think she could fix things only to then completely f*ck her up in the end.

Waiting until their wedding anniversary to reveal that he decided to marry her sister instead and show her a few videos (which did contain s*x but also importantly shitting all over all of their other memories together)

Then her sister goes ''Sure, c*nt move of us but you have it coming.''

The dude goes to himself that sure, he was just as bad as her and it was not the reasonable thing to do but he could not let go of his feelings of love and hate any other way.

Which honestly... not really. Like sure, it's definitly not the reasonable thing to do since revenge there is pointless but he is not just as bad since A, she did it when they were in a proper relationship while he just did it in some revenge scheme that she did indeed deserve (though it is, like I already said pointless) and B, it specifically mentioned that in that video he and the chick sister looked just like the video of the cheating wife at the start, just to s*umbags lost in pleasure and nothing else. But that was just them hamming it up to hurt the wife more, they very much so actually want to marry each other, it was not just some casual s*x stuff.