I haven't read the whole thread, but I noticed OP doesn't mention reproduction.
People act like they don't know where babies come from. (Hint: it's sex)
They can pretend all they want, but most people still don't choose sexual partners who are too young to reproduce, too old to reproduce, or of the same gender. Is it supposed to be a coincidence that people generally don't want to have sex with anyone unless it's the type of person they can make a baby with?
Not all sex makes babies, but all babies come from sexual reproduction between one man and one woman. That's why marriage is all about where babies come from.
Monogamous marriage is society's only way of helping babies have both their parents and encouraging people to build families for the benefit of the babies they're making.
Of course if you don't care about what happens to children, then you can pretend that sex is just a political weapon.
Very good thinking, I agree with you.
In fact, marriage is very important in all world religions, holy books etc. as the most important social unit and first institution created by God.
Nobody can deny it.
Today, we see other trend in many societies, that marriage can be between 2 man or 2 women but, in this way, society neglect the main purpose of marriage- human reproduction.
