Some more info on my situation...
I've been reading some books that my wife has purchased, such as "Open" and "The Marriage Makeover". I'm honestly still on the fence about an open marriage and have certainly not ruled it out. But the way I see her relationship with this other man is that they were college boyfriend/girlfriend. She transferred to another school and they broke up. That was well over 15 years ago. Now they've reconnected (yeah, thanks alot stupid facebook) and I find out that he's not had another real woman in his life since my wife, and he's never gotten over her. From what I've seen, they are in love with each other, and she wants to carry on this relationship via an open marriage. I might be ok with us involving others in our sex life, but I don't think I'm OK with her having a separate life-partner where if I was out of the picture they'd probably end up getting married and having a monogamous life.
Is this a normal reaction on my behalf? Are there different levels of open marriage, as set by the rules that I keep reading must be set? What if either my wife or I don't like the third person?