My wife and I had a relationship with her best friend. It lasted for over 3 decades. My wife and I both fell in love with her. We eventually went our separate ways when we retired but miss her. Sex is not the same. We got used to having a third around. Made sex and everything else more interesting and exciting. Hard to explain.
My wife does not know what jealousy feels like and neither does our girlfriend. Most of our marriage was in threesomes. My wife said it again last night, threesomes were our normal sex and she thought nothing of it. It was just our regular life. Even when our girlfriend went home to spend time with her husband, the sex between my wife and I was sparked by what we did the previous week. Our gf provided both my wife and I with things that we could not provide each other and we in turn provided her what she needed.
Only now that we are no longer together, do I appreciate how rare and wonderful our relationship was. While we were living it, we took it for granted. My wife still wants another women to join us. The attraction to a single women is that my wife is bisexual and also enjoys watching me with other women. A single bi female who likes sex with both of us, is a perfect fit for us. In return we accept her into our marriage and provide her a family that loves her.