If you told him that you still wanted to be with him, but not while he's with her, he could be thinking that he could possibly have the best of both worlds, and is trying to get in a position where he could maintain some type of relationship with you, intimate or otherwise. Sometimes this is simply a way of keeping 'control' of the other person, or perhaps he is afraid if he doesn't keep in contact with you, he will lose contact altogether. It is possible that he values your friendship that much, but the botXXXXX XXXXXne is if he cared/cares about you then he should respect you, and what he's doing is disrespectful.
You could mention to him that what he's doing is borderline stalking and that if he doesn't stop, then you will have to consider taking out a harassment/restraining order. Consider that if you've never met this girl, that it could a "fake" relationship set up in the hopes of making you jealous. Also, if he won't stop, even after threatening him with an order, then you have to wonder if his 'obsession' will get worse. If he will/can not stop, or things start to get worse, its important that you think about your safety before anything else. Always follow you gut feeling and if it feels like something is wrong, then it usually is. Call the police and sort it out later. Even if it turns out to be nothing, its better to be safe than sorry.
I welcome your thoughts, let me know if you want to talk more
Have you seen the girlfriend? Are you confident of her existance? How did you find out that it was serious (like heading to the alter)?
I'm wondering how much a part of you is punishing him for breaking up with you...and how much that had to do with your conversion. You've given a lot of thought to the situation, but it's extremely important to question our own motives at any given time.
Thru out history there have been couples who haven't necessarily fit type. It's possible that he may feel more comfortable and safe with her. There's no 'competition' because they are totally different. When it comes to love/sex/romance, there's not always a 'league', it goes beyond that. Think about when you see a couple that seem so at odds with each other, but have been together for years.
If you think he is your ideal mate, perhaps you should let go and tell him that. Admit that you made a mistake in breaking up with him and that you want him back, but not while he's with her. If he won't break up with him, i believe your initial plan is best, XXXXX XXXXX no friendship.
I'm here if you want to talk more