Wow....my heart goes out to you man. This is a very crushing situation. This could have happened to my husband and I, heres how it went down for us. I asked him to be poly, we actually werent doing that hot (i wont go into it) and I had strong feelings for a good friend of mine, but I did not want to leave my husband (my friend is married, hubs and I have kids). While hubs decided if he'd stay with me or not, because I told him i was poly with or without him and it wasnt fair to him for me to force mono, BF and i discussed exactly what you are saying with you and GF. If my husband had left, I would have wanted so much from BF because I didnt have a primary anymore, and he has a wife to support so he could only be 'so much' of a primary to me. I was terrified of trying to find a primary while also dating someone already.
Here are stories diverge, as i was lucky that my hubs decided to stay and I didnt have to go through that, but my point is I know what you are feeling.
As to your wife, it sounds to me like perhaps she was never poly in the first place? As much as I adore BF, i would not have left hubs for him except maybe at some of my lowest points but I was able to work through those 1)for my kids and 2)because BF was married. I also caqme to terms with the fact that mono just isnt for me.
SO how do you decide? Picture yourself when you were mono...or being mono with someone you deeply care about...and if you'd still feelign the urge for outside love/sex. I know i would. I also reccomend reading the book The Ethical Slut and checking out the website morethantwo.com
good luck, and I hope things turn out for the best for you
keep on keeping on