Exactly! America is one of the few nations that still upholds a general societal regard towards monogamy and yet we have a 50% divorce rate!
I'm interested in this statement .. what other countries are you speaking of that do not "upholds a general societal regard towards monogamy" ?
I have no problem with another couples interest in an open marriage, even though I do have strong spiritual beliefs. I wouldn't judge them at all. However I don't see this working with many couples, as it definitely undermines the security part of being in a marriage. I do know couples that thought this was a good idea, and in the end one of them just wasn't as committed to the relationship as the other and were really just keeping their options open.
Even in cultures that support multi partners, marriage is usually in the picture, as in one person is married to many people. We see that usually with multi wives, but I've read about cultures where the woman has multi husbands, but again, there is a secure arrangement/committment involved.
I think the bigger picture is not in our religious convictions, but in our mammal insticts. To be part of a tribe or 'pack' brings security. If the tribe is infiltrated then security is jeopardized. Being in a modern society does not nullify our survival instincts, although we like to believe we are above all of that.
Marriage is a pact of sorts so I guess the question is, what sort of pact are you entering, and why enter one at all if you are not interested in being in one? What is your motive for entering the pact? Single is a perfectly viable option where you can have as many partners as you want.