in response to david's note about marriage
In addition to most of what you've said, it's also occurred to me that maybe marriage as an institution and a construct is useful because it essentially is a mutually agreed upon raising of the efforts required to get out of a relationship. In other words, I mean there are the very mundane but time consuming aspects of a divorce (legal fees, contacting a lawyer, custody, property, etc), but they're really only important as extensions of the larger idea of a merging of two people. I know we're all bigger on the emotional/psychological/spiritual dimensions of such a conjoining (oh, and the physical... although that's not at all what i'm talking about, but since I'm a guy I guess I had to throw it in) and with respect to the emotional stuff we think: why the hell do we need marriage if it's just a technical affirmation of what we already know in a more visceral meaningful way? I mean, the short answer is we don't, but I guess the long answer might be that why shouldn't that emotional/spiritual/whatever conjoining also have its legal and socially constructed counterparts. Or from another angle entirely, if I and my partner mutually agree that we'd like to be together for the indefinite future, conceivably even the duration of our natural lives (because when I am reborn as a robot, I want to be able to date other hot robot chicks--not my old repulsively human wife), then why not make it such that should an extemely difficult but ultimately resolvable conflict arise between us, we have additional incentives to work things out, and should we decide not to, then it would be easier for me to feel certain it was too much to deal with, because otherwise why the hell would I think it worthwhile to go through the double hell of emotional devestation and pains in the ass that both come with divorce.
Also, with regards to marriage and monogamy both being social constructs I agree, with regards to whether or not they bring a semblance of order I'm not as sure, just because the whole idea of their being mere constructs may be that there is nothing organic or intrinsically inevitable about monogamy or marriage, and so if society were constructed such that we all didn't have this big hang up about our soulmates of opposite gender neccesarily being our exclusive bedmates (or more importantly our being THEIR exclusive bedmates), and maybe... instead sex were more divided from emotions, or both sex and emotions were made communal and inclusive rather than competitive and exclusive (one drawback is of course the matter of who would raise the world's children and how) then maybe we'd all be cool with that too.
Having said all of this, and I'm not sure how much any of it's worth, I still desire that happy day when I can convince myself to marry someone (or much more importantly, consequently convince them to marry me) because of all manner of half-imbibed dreams, stories, and images of marital happiness which, in sum total, basically just means in my limited myopic way I try to see outside the constructs but rarely actually manage to sucessfully divorce them from my inner self--like i said, it's just such a pain in the ass.