At this point I don’t think anyone has done anything wrong.
A bit of flirting at work I’d say is normal, even when married, it’s just the human condition. It’s when things go beyond that it may be time to stop it and people become guilty.
In this story there is no culprit. Heck, I read it as maybe he picked up on it and decided to stop himself cold turkey. I wouldn’t hold it against him, if anything that is an honorable trait, to self reflect and realize it was heading in a bad direction. If I were him, I probably would have knee-jerk reacted too if I realized it. Yeah it might be nice to talk about it… but work complicates that and it opens the door for HR and a whole bunch of trouble. It also could have just been you read into it a bit too much, I can’t say I wasn’t there.
There’s no harm in developing a little crush, even one on someone we shouldn’t have one on. The harm is to follow through with it, or to let it linger. I’d say this is a good time to reflect on it, admit it was fun, but move on professionally together. I would be courteous to him, friendly even if you see him in the break room, but this is the time that I’d recommend coming to terms with it’s just a silly crush, nothing will happen with it, and to look elsewhere. No guilt, no blame, it was fun to imagine, but it’s not healthy to dwell either.