Marriage defines the relations and obligations that form the core of family. These concepts are vital to how we relate to each other and the people around us. There is nothing really new here. The system of laws that govern marriage just codify the common practice that was prevalent in the older smaller societies the precede the modern one we live in. The laws were needed to enforce in a large society the practices the smaller immediate community would have followed in a previous age. One important function of the law was to normalize different traditions to create a blended culture workable for the whole.
The traditional relationships defined by marriage are between parents and then between them and their children. The privileges and obligations defined support the creation and success of families. There are other relationships involved, but these serve the same goal. All of this implies that the main function of the system is child-rearing. I believe that function is still important, but not to be considered a defining element of a marriage. The important parts are how we relate to and support each other. It is best when we follow our own best instincts, but the law is a useful backup to resolve conflicts.
Religion was an important aspect in the traditional communities that first defined marriage. The traditions and practices of individual communities are important. They define the community and the composition and expectations of the members. Some communities believe that marriage should be limited to one man and one woman. They cite the teachings of their religion as justification for the restriction. I agree that such a restriction is valid within their community. Problems occur when they try to impose such restrictions to people outside of their community of faith.
Bodies of faith are important parts of our lives. They tend to be very personal. We self-organize with other people that have similar thoughts and ideas. This is important two ways. It helps to associate with people who share our beliefs. That association connects us to something outside of ourselves. Because of that we are free to explore parts of our lives that we might not share with anyone else. Through that sharing we find things that we might not agree on. A safe place to discuss small differences is an important part of a body of faith. A common core of beliefs is a good place to begin discussion about things that we might disagree on. We can safely disagree about many parts of our lives if there is some shared bond.
We often find things that we disagree on. This happens between people and between bodies of faith. Too often we pay more attention to the things we disagree on then to those where we agree. That is unfortunate. Our greater society is strongest where we have focused our attention on points of agreement and found compromise to resolve disagreements. We can still disagree, but we can work within a shared framework that encompasses our beliefs.
Marriage is a term that was defined in the context of religion. It's important civil aspects were made part of civil law. The collection of contracts that are part of that civil law are important to all of us. Many people talk about marriage from the point of view of their religion. Modern society is made up of many religions and bodies of faith. It is not necessary that they all agree on the parts of marriage related strictly to civil law. It might be time to call the civil law part of marriage something else. That would leave it to your own religion to define what marriage means.