In terms of why can't people be free to love other people, what works for you may not work for others. You describe a very open arrangement with your lover. I know for a fact that such an arrangement would not work for the majority of people. Why? Because people are selfish and people get jealous and despite wanting to stray at times, they seldom want to share themselves. You alluded to this yourself when you said
Look, don't misunderstand, I'm not advocating an open relationship for everyone. People are different and have different needs in terms of their relationships. I am simply saying that people should be real with each other in terms of what they need and establish relationships based on those needs. And, I am saying that changes in relationships are inevitable. People change. It's what we do. It would be great, (maybe,) if people stayed the same for all of their lives but they don't and so it follows that relationships between people must be fluid in order to work. Is love the most important component in a relationship between mates or is fidelity more important? I was married years ago. I love my wife -- still, to this day. She loves me -- still, to this day. We are not married anymore (and I feel compelled to add here that infidelity had nothing whatsoever to do with the breakup.) We changed as people and, as a result of the changes, found that in order to preserve our love for each other, we needed to be apart, so we divorced. If she had come to me when we were married and said, "Tim, I love you but I need to be with some other men for a while." My response, I hope, I trust, would have been, "I love you too and I support your need to do this because I love you but this if your thing and I don't need to know about it."
I think a lot of the times the need for sex outside of marriage has absolutely nothing to do with the relationship between the people involved in the marriage. To end a love match over that seems to me like throwing the baby out with the bath-water. If it does have to do with the marriage, I think probably that the people involved are either missmatched or they are not mature enough to be honest with each other and perhaps should look into divorcing.