I cannot possibly cover everything to be said here
I'be been badgering you on other threads in other sections, but I thought I'd do it here too as I haven't engaged you yet on the roots of your beliefs 
But first!
Historically, those societies that have moved from this (Romans and greeks) have fallen.
:wtf: That has to be the non-sequitur of the day...if you even purported to be rational in the first place. First, the Romans conquered the Greeks. Secondly, the Romans for the most part practiced monogamy (complete with rituals and legal divorce). If anything, their inconsistency lay in the religion. And their primary cause of collapse was that they became fragmented and extended their borders so far that the integrity of their defense and military infrastructure was severely compromised. Add the typical inherent political bickering, infighting, fall of the republic, and the social upheaval that accompanied Constantine and bang, there go the Turks. IMO that doesn't have a whole lot to do with "failure to adhere to the ideal of marriage between a man and a woman".
So far I've seen that you have a strong support for the stability of tradition, as well as presenting a unifocally traditional Christian perspective. When it comes to a discussion on what marriage is, your perspective is therefore limited to the Christian definition, which thereby might restrict the import of your commentary on this thead because we are not dealing solely with Christian precepts. i.e. you're entitled to your beliefs but in rigidly subscribing to a single framework you necessarily limit yourself.
I can appreciate that people desire structure. But I'm going to contend that adherence to tradition is far more harmful than it is good, especially when traditions are based on misconceptions. We cannot assume that the knowledge we have is complete and even if you presume revelation from a perfect God, we can only deal with interpretations of such, and those are invariably flawed. Historically, the original concepts that seem so fanatically argued upon but are only peripherally related to the actual central tenets of the one holy, apostolic catholic church, were those that borne from a completely different context. Times change, and their meaning becomes distorted. In the end, the traditions you support will be as empty as the debates on what rules to impose and which group of people to persecute next. As you say, themes and central concerns remain common between denominations. Due to the awareness of changing times and the subjective nature of interpretation, central doctrine is constantly reformed and reviewed and has been ever since scholars in the 1st century AD.
At this point, I would like to raise a theological quibble. I presume that faith is different from religion, in that religion is the practice of a faith. That religion is institutionalised such that it is practiced by many in one manner (to a certain extent) means that it is governed by traditions. Therefore I really don't know if your statement "it's not a religious thing, it's a tradition thing" has any meaning. I already explained the active process of review and reform behind the doctrine common to all denominations of the one religion anyhow.
As for the actual topic at hand, socially and behaviorally, we have been structured for quite a while such that monogamy is the most stable and satisfactory type of relationship here. I have a problem with your implying that this is in any way "natural" though, not least because by our current understanding of what "natural" entails, it seems that this really isn't the case. That only goes back to highlighting the distortion of our beliefs and behaviors away from the parameters of our biological existence...which is in turn why we need to constantly examine our beliefs and traditions!
If you'd be so kind as to explain what you mean by "natural order of things", we could begin a discussion on the problems of appealing to nature. I'll start by saying you presume too much. But maybe I might have to begin a new thread on that one.
