I am currently in a six year poly relationship with my wife, and going on two years with my girlfriend. My girlfriend does have a separate relationship with a guy that has been going for about a year.
I have personally made all the mistake a man can make learning about poly, in various relationships over the years. One of the earliest things I learned was (for me) it seems that trying to restrain someone else's love for a new person will most likely end up backfiring on you at a later date. Learn to enjoy their newfound joy.