To get down to the root of it would take back to sex. More people marry because they don't want to find a new sex partner at the time. Than those who truly love each other. So maybe it the sanctity of sex more than anything else. I have known too many guys who married some chick they found in a bar, went home had with and had sex with her. The next thing you know they are living together, then they're married. After two kids or so now they want a divorce.
I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago.
Edgar Allan Poe