Abaddon,
You make some fair points although you really narrow the application of your reasoning to only gay marriage; and it is in that sense that your reasoning becomes irrelevant to the larger destiny of marriage and family itself.
The bombshell paper Thi Chi posted eariler on this thread was all about "After Gay Marriage" and its eventual dissolution by major law scientists. This falls right in line with the overall philosophy I experienced in University studying marriage and family some 7 or 8 years ago.
That basic reasoning is founded upon the premise that major caretakers and a few moden family structures no longer need to be limited to parents and children but can and should include any number of relationships bound by a very loose criteria. My professor agreed that under the criteria she was presenting to us that some crime families or gangs would qualify. Because, they among other things were emotionally interdependant, shared common resources, lived together, and shared duties common to the functioniong of that unit of humans etc. Sounds incredible, but that is what she conceded to upon my questioning.
While some lawmakers may find it necessary to expand definitions of human relationships to accomodate real and actual needs to protect the vurnerable, the net result of the more radical legislative efforts will inevitable take society toward a secular existence.... and to that end gays may end up with no marriage at all if it is dissolved; but still accomplish the goal of emotional stasis in the absence of the ideal of one man and one woman marriage. So, it follows that the very existence of hetero marraige as a societal ideal is the real enemy of all alternate lifestyles. Once, that is gone, marraige will no longer be needed nor desired because it will have lost its "deep symbolism".
Only the lawyers will be left.