I don’t believe that marriage is inherently religious. In addition to being a well-defined legal status, on its fundamental level it is established and maintained by two human beings making promises to each other, by society recognizing those promises, and by the individuals keeping those promises. Religious beliefs about marriage are ancillary and often contradictory.
Marriage can mean whatever you want it to mean. I am simply saying that it originated as a divine religious principle and has been followed through the ages as such. Only recently have atheists decided to use it as some kind of social symbol of sorts.
by the way, are you claiming that Adam and Eve are historical people and that marriage began with them?
It doesn't matter if they really existed or not. The earliest recorded histories define marriage as a divine concept sanctioned by God. This was always understood as such until recent times. It is too difficult to discard such an integral element of human society, so atheists just adapted to it and tried to squeeze out the religious basis for it by pushing for legislation that said non-religious persons could administer it.
The atheist who are trying to outlaw Christmas because of its religious heritage, should also reject the concept of marriage. The whole idea of faithfulness to one's spouse is a religious grounded principle that has been accepted through the ages. Atheists just don't like to admit it is based in religion concepts.
I doubt the cave men were devoted to one wife. More likely, they went about raping anything that looked female. At some point in time, something in human evolution/civilization had to establish a rule that said a man should be faithful to a woman and support her throughout his life; likewise for the woman towards the man. The earliest hint of this being established is found in religious texts when scripture authors claimed God established this rule.
“All knowledge of reality starts from experience and ends in it...Propositions arrived at by purely logical means are completely empty as regards reality." - Albert Einstein