Killarney,
I know how you feel. I, too, had a very serious college boyfriend whom I still think about every once in a while. Even though I love my current boyfriend very much, and know that he is the person with whom I will most likely spend the rest of my life, I dread the day that I hear my old college guy is getting married.
I know that news will be tough to hear, and that it will probably be quite upsetting. He was my first love, and to think of him with someone else seems wrong somehow -- even though I fully realize that I'm in a much better relationship now and am happier than he could have ever made me. It's hard to explain or rationalize. Just know that many of us feel the same way, and that you're not the only one!