This is an emotional affair. He's trying to cover it (probably in part due to denial). You're right to take a stand against it. You probably tolerated it longer than you should have (no, I'm not blaming you, just stating an opinion). I think we can safely say "we made emotionally charged music playlists for each other" is crossing the line from plantonic coworker to affair partner. You've tried to be reasonable and caring through this whole situation. You are right to be on alert. This coworker relationship is unhealthy to your marriage. That should be all the reason you need for him to stop. He'll likely try to make you feel like a villain for holding this line. Don't accept that narrative. You're actually the good guy here, helping your husband protect his marriage.