Great article, James. I believed in the same reasons as your friend until I started to work on myself. I have gone through a situation with someone I was interested in. He had a ton of friends from the opposite sex which made me slightly uncomfortable. It was definitely rooted in my own insecurities which I'm still working through. It definitely comes down to the emotional maturity/self-control and the boundaries to develop security. From the friend who cut you off, I would think it happened due to the insecurity of the person she married. Honestly, if you are secure within yourself and see yourself as "the prize" then, although it will hurt, you'll be able to appreciate the experience you had with that person and accept the situation for what it is and let that person go if it doesn't work out. When you're secure you don't feel the need to control the friendships of your partner. Mature adults should be able to have a healthy relationship and keep their friends. People are going to make their own choices whether they cheat with a close friend or a stranger so I don't believe in cutting out a friend to avoid temptation. Insecurities definitely need to be discussed and worked through.