Well, I'm not big on relationship advice so I'm not going to give any. Instead, I'll say how I feel. Women and Men, in general, have very different priorities. Men seek loyalty. Most of us want a woman that will be loyal no matter what. On the other hand, most women seem to think that loyalty takes a second priority compared to being happy. I personally believe that a marriage is a sacred vow. I can understand the anger, but anger is not useful in this situation. Talking it out is the only thing that may help. However, I really don't like the "cut off" comment. That is just wrong. I would have no problem with my girlfriend/wife confiding in a friend about our relationship. However, there is something wrong if that friend becomes closer than the significant other. Love is patient and forgiving. If two people truly love each other, they can work through any difficulty.
As for Isla and GF, I agree that a healthy relationship has no need to fear your other flirting or talking to the opposite sex. However, there is a still a line that must not be crossed. I believe GF expressed that in her post about theoretically kicking her man out if he went that far. I wish them the best, but I think there will be problems. The "cut off" statement shows a very serious problem. You dont' do that to someone you love.
-Dagta
P.S. If this is unintelligible, please excuse me, i'm very tired