Watch her confess BEFORE him, even if she kissed Niina like twice and You went all the way to his coworker....
And yet, it's still a fair argument to say that her cheating was at least just as bad. Relationships are not just defined by physicality, they are defined by commitment and dedication to the other person. Emotional cheating is maybe harder to define and pinpoint than just a physical act, but it can be much larger of a dealbreaker than sex, especially to women, as we can see with Kaede - she can forgive lust, but it's much harder when the love is leaking out.
Besides, that's not a competition. The point of their relationship now is to show and contrast two sides of cheating - physical with no emotions and emotional without physicality. It's almost certain they will both be informed about the other's infidelity, and the focus will be on how will they get there, how will they fare along the way (guilt, self-loathing, doubt) and how will they deal with it after.