So...

[ShadowHM]

No, those are both fine situations to be married as far as I'm concerned (provided that the love and commitment is there, of course).  They are also carefully chosen exceptions.  Marriages like these are both significant and justified, but it is the role of marriage within the family structure that I think makes it an essential institution to society. 

First of all, Nystul, if they are both fine situations to be married, then why did you place the caveat in your first post?

husband and wife who wish to devote their entire lives together and raise a family

I queried you because I have seen all too many who carry on about the importance of the raising of children and how it is the central part of 'marriage' but then equivocate in the same way you just did. If you have moral issues about a marriage between homosexuals, don't dress them up in terms that side-step the issue.

Further, the first situation is precisely the one that my mother is in. She and my 'step-father' have chosen NOT to be 'legally married' because of the legal ramifications of that ceremony. In the eyes of both of their families and of society in general, they are married. Legally, they are not, because both of them have problems with the way the current laws about marriage insist that the surviving spouse get at least half of the possessions. Both of them are quite insistent that their own children should get the fruits of the labours of their now-dead parents and not have it diluted by a second spouse. They have very carefully structured their lives to make sure this will be so. In no way does this diminish their love and commitment to one another or the love and admiration that their friends, relatives and 'society in general' have for them. I would like to note that this was a tough decision for a woman who has been a devout Christian for her whole life.

I suggest that the existence of bad marriages is not an excuse to allow more bad marriages.

You still have not explained why it is a bad thing for society for homosexuals to make a commitment in the form of marriage to one another.

The existence of good marriages in society is a positive thing for all to see and witness. It strengthens all the bonds that hold a society together. The hallmarks of a 'good marriage' are love, respect and (above all) commitment to one another, in my opinion. What else is there, in the long run?

As to the main thrust of the question: Will it happen in the U.S. of A.? You are probably right that it is political suicide to support it.

I am not so sure about the splintering effect it will have on the Christian sects. There will be considerable and spirited debate, no doubt. But there really is a limited spectrum of choices available, and there will be places to go for anyone who finds him/herself a dissident within their current congregation.