Except it hasn't existed as 1 man + 1 woman for 4,000 years. Polygamy used to be a lot more common. Marriage between first cousins was (and in some places still is) standard. People choosing their own marriage partner is really recent, i.e. only in the last 200 years. American women not being considered the legal charges of their husbands only dates from the 1920s (at the earliest). My point is that it has changed a lot. I don't know why people keep repeating this claim that it hasn't changed in 4,000 years. It just isn't true. Even now, if you look at different parts of the world, marriage is practiced very differently from one place to another, and even between different social classes in the same geographic location.