Thank you for sharing this context and I entirely agree with you. I've had several of these type of guys message me (literally SEVERAL) and tell me their girlfriends/wives wouldn't know. I had one guy call me on the eve of his wedding to tell me he loved me...and this was a guy who had an entire measuring contest with my oblivoius husband who said, "Yeah, he seems really accomplished and great!" My sister was like, "Um..."
I wish more men recognized and/or took responsibility for how inappropriate this is.
Sadly, every single man who did this with me is still with the partner they emotionally cheated on. I was never friends with the girl, so I feel like it's not my place to say anything.