Honestly, this confrontation was written about as well as I could have reasonably hoped.
It was satisfying that Kayo revealed everything so easily, because beating around the bush at this point would be aggravating. And it's nice that Yusuke specifically noted that no matter what she says, he thinks she'd just cheat again eventually. (Though I would have liked for him to specifically call out her lack of trustworthiness.) Ultimately, she cheated on him not for any acute reason, not in a moment of stress or doubt, but merely because she felt horny for a cool guy (or was hooked on the feeling of validation, but that doesn't seem to totally track with what we know about her), and she only stopped because she got spooked. I don't think that's poorly written; some people do just be like that. If she were forgiven, then the moment the fear fades, the moment the guilt fades, the moment the urgency of this confrontation fades, then she'd be right back to relying solely on her own willpower and loyalty, which she's already demonstrated to be weak as shit by folding TWICE. And that's IF she's sincere, which is doubtful because there's every indication that she only came clean because it potentially benefited her, not because she actually cares that she hurt Yusuke and violated his trust.
Ultimately Kayo seemed destined to be mostly just a prop to get the main duo together for the romcom, so I guess I'm glad she was at least a coherent character. Extremely selfish and shortsighted, but coherent. I know the manga ends soon, but I personally would have been interested in seeing what happens to her character. Obviously the premise works better if she stays a bad person, but I also think that continuing to beat up on a villain after their defeat is kinda lame. I'm all for seeing how a character changes after they hit rock bottom. Oh well. 🤷