You started off alright with this part:
Yes, I would say this is at least largely true.
I'm no anthropologist or sociologist, but I would wager that there are certain characteristics and morals that are common to all societies and cultures, such as that taking someone's life without good reason is condemned. What's construed as a good reason will vary between societies, of course.
I think you're going a bit too far there. They still have the same hardware and evolutionary programming because they're part of the same species. That contributes a lot. Culture isn't all any given person is; they're also biology.
Now this is where you lost me completely. Because there are, and have historically been, societies with differing, to various degrees, characteristics, all human experience is meaningless? Did I understand that correctly? If so, how did you arrive at that conclusion? It reminds me of the line of reasoning that because you will die one day, nothing matters. The present experience matters. All experience matters. It matters where it happens and when it happens. It matters right at that moment.
We're all atoms. Does that mean we're *just* atoms? Everything that I am consists of neural configurations. Yet I am more than those neurons, am I not? Love is a chemical reaction, but we don't look at things on that level unless we're analyzing brain chemistry. Moreover, love is an emotion, a biological function, not a social construct. Marriage, and arguably/possibly homogamy, are social constructs.
If it's arbitrary as you say, then how is it possible that there is any order in society?
