Who started the concept of marriage?

Religion is why most people feel they should get married. Others just want that everlasting bond with the person they love. For some religious people, they believe that marriage must come before many parts of life: sex, children, and even sharing a living space. Unfortunately for many people (often people that are too young) marry to soon in a relationship just to get the "benefits." They feel that marriage is the only way to have a joined life with another person. So where did we get this idea that we have to be legally and spiritually (if you will) bonded to a person before we can have a life with them and participate in activities such as living together. I believe that a person should most defiantly share a space with their significant other before marriage, going into a house together after never spending more than a few days with that person can be stressful and may uncover some shocking living habits. 

So is getting married "natural" meaning are humans meant to do it? Well, the concept was created by humans or God (again, if you will). This is where things get tricky, if you stand from a religious point of view you may say "it is in the bible, so that is why people get married, and should." Although those who do not follow a religion may say that they are in it for the binding legal document and just have the ceremony as a perk. I myself am a religious person, but not to any extremes such as believing any of the things you "can not do before marriage" well maybe having children for the reason that no one is financially or mentally ready for a child until a more stable point in their life. This point is usually when you have a mother and a father for the child that love each other and are a stable family unit. So marriage is not natural in the way that a forest is natural, but in more of a "humans created it so it is natural" kind of way. 

Question: What is your stance on couples, having children and living out their life like any other person, just not ever marrying by choice? Is this "natural"?