I used to think this and remember my friends and I starting to have this idea that marriage is a social construct in high school and college as we tried on our philosopher hats. A little later I read an interesting book called Sperm Wars [1] that largely tried to line up the results of scientific papers and behavioral surveys with attitudes and narratives about sex. They had what was a compelling argument for me that the cultural ideas behind monogamy are a social rationalization for what we’d be largely doing anyway. In other words, they argued that from as far as they could tell, we tend to have serially monogamous pairings, and the culture and religion views on marriage are just after-the-fact explanations for it, not the primary cause of it. There are, of course, lots of other species on our planet that also tend toward serial monogamy, and don’t have the same social constructs that humans do.