I find the idea that marriage is "primarily a mechanism for raising children" reminiscent of those absurd sociological theories that ascribe everything to reproductive drives. As if no other process were ongoing in the individual human psyche or within society as a collective taker of actions. This, and many other simplistic rationalizations that try to class people in such a way as to deny them benefits, are despicable and corrosive behaviors; direct affronts to liberty. For those of you familiar with my approach to life, you know that when something lands in that last category, I'm not going to take it lightly.
Marriage has been, and is being, used to: bind property, bind nations, bind previously disjoint families and titles, provide mutual support for the parties involved, latch onto property that would otherwise be out of reach, maintain a constant source of sexual relief, obtain financial support, obtain relief from a single lifestyle, avoid censure by the religious and/or sexually constipated, obtain citizenship, social climb, obtain green cards or otherwise allow access to one country or another, to obtain insurance obligations and pay-outs, to differentiate tax responsibilities, and... oh yes... as a structure within which one can, if one is so motivated, raise children. Of course, as you don't actually need marriage to do that, nor do you have to have children just because you are married — while you may very well need marriage to do many, if not all, of the above tasks — child-rearing is really a minor point.
One consequence of gays being denied marriage rights is to marginalize them, to ensure that they are not able to access the broad spectrum of social and personal benefits that marriage can, under various circumstances, confer. There's nothing stopping a gay couple from raising children without marriage (and it is worth pointing out that in the US, at least, presently the number of unmarried couples outnumbers the married anyway.)
Part of the problem — and I definitely think this is a problem for gays — is the tendency to turn to your neighbor for approval; if gays expect recognition, per se, from neighbors who abhor their lifestyle, they're going to have to wait quite some time. Specifically until society turns over enough mores to simply forget that it's not usual or OK to accept gays; just like it is still an ongoing process for women to be accepted throughout the workplace (for instance, speaker of the house is the closest (3 away) any female has gotten to the presidency as of early 2007... [The 2000 election brought that to 2 steps away] other stats agree.) Acceptance is increasing, and enough pressure seems to be there such that I think it will keep increasing overall. But it is early enough in the process that you can expect major attempts at accelerating that acceptance to meet strong resistance and appear to constitute major setbacks.
The funny thing is, they're not really setbacks. Every time the issue raises its head, homophobes feel like they're on the verge of losing control — and they're right. Every time the issue raises its head, homosexuals feel like their issues are being heard — and they're right, too.
Every time the issue raises its head, the undecided are forced to confront basic issues of liberty and equality, and this is a process that will almost always favor the oppressed, in this case, the homosexuals — in my opinion.
These are part of the social pressures that are most likely to eventually marginalize the homophobes. Just don't hold your breath. Like religion, homophobia is a deep-seated social problem that can't just be surgically excised; it is systemic, it is sub-rosa (in the sense that most homophobes cannot articulate just what it is that is causing them such distress about the idea... it is secret even from them), and it is insidious in that it is a fulcrum used by the vicious-minded to artificially create social divisions at every level from cliques to classing (like marriage) to corporate assignments to political positions.
We're not going to solve this here, but we can apply just a tiny bit more pressure. One way to do that is to expose class-restricted marriage for what it presently is in the USA, and that is a government sponsored gateway to a set of conditions that mostly, with the exception of taxes at present (see "marriage penalty"), confer benefits upon the recipients of government permission.
From sharing medical benefits to receipt of insurance pay-outs to the ability to visit a loved one in the hospital, marriage is a cultural gateway to concrete benefits managed by the federal government. Every day that passes during which these benefits are arbitrarily limited to single-partner, opposite sex pairs is a day that undermines the commitment of our government to liberty and equality, and a day of continued malfeasance for those citizens and public officials who support such an outlook.
Perhaps when we can get this done, we can begin to seriously look at the paranoia that causes polygamy and other arrangements of consensual, informed personal choice to be looked upon with such fear and loathing, and excise those social problems as well. I hope so.