I have to agree with you... but I need to go a bit further.
I think that all people who link together and cherish not just who they've managed to find -- but the beauty of linkage itself -- tend to want to enshrine to themselves and before the eyes of the world, the beauty of that linkage.
So I see two main factors here: one, the importance of linkage ceremonies. But I also see the importance of what I've stressed before, in my essay called Controlling Language, that we need to control our vocabulary and assign certain terms to mean very specific things... otherwise, there will be confusion on very important things, as is the case here.
When we use the term "marriage", we may need to know precisely what it means.
Your argument -- as is the argument of many others -- that the term "marriage" should apply only to linkages with the very specific potential to actually breed children, seems valid to me... if I understand you correctly, that is.
Now, as I see it, there are linkages for the sake of emotional and physical support and sustenance, and linkages for the sake of doing something else. We create terminological variations on the concept of "marriage" to apply to situations outside of intimacy: we refer to corporate marriages as "mergers", but also "marriages". If two people of prominence in their respective fields collaborate to accomplish a mutual goal, we call that a "collaboration", but also a "marriage".
The interesting thing to me, is why I see so few people having an issue with the hijacking of the term "marriage" in those instances. Hell, even Reese's Peanut Butter Cups are the "marriage" of chocolate and peanut butter.
But back to the two-person issue... Should it be that important to differentiate between a potential breeding and child-rearing union, versus all other unions? Oh, I definitely think so. Despite the fact that there are even many married couples who think of child-bearing and raising as a "hateful duty" (a la Ayn Rand's own mother), it is still probably the most critically important human endeavor there is. In my eyes, that needs to be sanctified to the nth degree.
I think that there should be an institution whereby the penalties for abandoning a child-bearing relationship should be heavy, and where the incredible importance of that union is reverentially stressed as something which should not be disturbed by outside Casanovas and Loreleis, lest the penalties also be severe.
But I also think that the other kind of linkage -- which exists for the fulfilling of non-reproductive, intra-personal, profoundly important needs -- is also crucial. Lonely, unappreciated people are unhealthy, miserable, disease-prone people... (believe me, I should know), and when you find somebody who finally and truly "gets" you after too many lonely, lonely years, you want to celebrate... and you want to proclaim the specialness of that linkage to all the world.
But that sort of thing can happen between two males, two females, or a male and a female.
In this day and age, however, where the fear-based social pressures are so great for males and females to "be together", I see that very coercion creating quite a lot of deep-seated animosity and contempt for the entire notion of heterosexuality... heterosexuality which would likely be a much more natural and pleasurable thing, if it weren't being so noxiously shoved down our throats all the time, and if the way we are told we're supposed to act as hetero males and hetero females, wasn't such a confining and contemptible disgrace.
Given the overall setup for heterosexuality -- and this is something I've said before -- my gut almost feels like someone has perverted it into something that they intended to be unpleasant and sado-masochistic... almost as though it was intended to foster homosexuality through resentment of said "heterosexuality", as an aversive set of conditions.
But I suppose even through all of this, that there are males and females who truly do come to like and even cherish each other, and even enjoy pulling each other's hair and saying "Give it to me" and "Who's your daddy?" behind closed doors. I have been there, and all I can say is that today's female seems far too dangerous, sadistic, and profoundly loveless and malicious, for me feel at ease any longer to even try for that oasis of sustenance.
At any rate, because there are so many pressures and aversions associated with the role of heterosexuality, I don't think you'll ever be able to make it disappear... and trying to "stamp it out" -- which, let's face it, is really what this is all about -- will only add fuel to the fire of resentment of heterosexuality, and as I see it, create more homosexuals. (This is how insanely dumb the militant hetero camp really is).
Instead, the only way to increase the popularity of heterosexuality, is for us as a society to make being heterosexual actually enjoyable again.
The role of "ideal hetero male" should be re-written so that he is no longer an amoral and degenerate, pussy-chasing conquistador who takes pleasure in leaving an accomplished and wide swath of destruction behind him. And the role of "ideal hetero female" should be re-written so that she is no longer a sadistic and litigious siren, hell-bent on the emotional, psychological, and financial destruction of any male she can "lure in" with her "feminine wiles".
This is a hellish situation for both males and females to have to conform to, and I don't give a rat's ass what anyone says in rebuttal... this is what drives people away from the entire institution of heterosexuality and marriage. And rightly so.
I have even heard several pastors on the radio, proclaim their resentment for homosexual unions, because those people are actually enjoying themselves. Their implicit argument is that marriage isn't supposed to be about pleasure... it's supposed to be a protracted ordeal... a miserable sacrifice... that two people must inescapably suffer through, for the sake of the eventual achievement of some warped sense of "character building" and "goodness". Their explicit argument is that "homosexual unions are wrong because they're about hedonistic pleasure". What rational person shouldn't cheer at such a prospect!?
But do you see the point? All of us implicitly understand that heterosexuality should mean misery, and who in their right mind actually wants something like that?
but if you change these things (the conditions of heterosexuality) and actually let them become natural and pleasurable as they were for each of us before social and religious mandates enslaved us, then you have something you can truly be proud of... and heteros will actually be enjoying themselves so much, that they won't give two shits what any homos that still exist, are doing! (To get a sense of such a forgotten state of bliss, rent the movie entitled The Blue Lagoon).
At that point, nobody should really give a flying damn who is getting married and who isn't.