This post makes me sick just thinking about it. I'm a firm believer of treating other guys with respect when it comes to their gals. If a woman leaves a guy for you, she will leave you for another guy eventually. Going after "taken" women doesnt' make you a DJ, it makes you a TOOL.
A few years ago I dated Miss Wyoming. She was pretty stunning. I remember when we would go out, be it a resturant, whatever, there would be silence when we walked in. It was a little uncomfortable actually, people would just stare. Not once, did I have a guy disrespect me by hitting on her when I was there though, thankfully.
Fortunately people like MrLuvr are rare...although, if it did happen to me, I would calmly watch my gal's reactions. How she reacts will tell you everything, if she's quality or not. Afterwords, go up to the guy and confront him privately.