In summarizing what has been posted so far, we seem to have two distinct sides in this issue; Those who believe it should be the right of same-sex partners to marry and those who don't consider same-sex partnerships to be marriage. Most people would feel comfortable agreeing they fit into one of those two viewpoints. There is no actual 'middling' here.
Now, what the gay marriage advocate believes is, this 'movement' is going in the direction which favors their view. The infamous March of Progressivism. They boast of same-sex marriage law in numerous states and growing, and this is true. However, this only brings to a head, the defining moment at which something has to be codified as part of the Constitution and ultimate law of the land. Whether that is like Women's Suffrage and through the Amendment process, or whether through an Act of Congress like the Civil Rights Act of 1964, the 'defining moment' is yet on the horizon... but it's coming.
Because this appears to be on it's way and nothing anyone can do about it, we see an often insulting level of smug confidence from gay marriage advocates. What they completely fail to realize is, this is an issue where the sheer numbers don't fall in their favor. They simply don't have the numbers needed to ensconce gay marriage into our constitutional lexicon at this time, and are not likely to have those numbers in the future.
It can be more closely identified with the ERA movement from the 70s, where the same smug confidence existed in progressives who burned their bras and cheered on Women's Rights. Now... where has that gone since it failed in the 70s? We've passed a series of law reforms and bills to ensure gender equality and fair treatment, but without having to radically change the constitution.
The most grim aspect of the details as to why Gay Marriage will not prevail, is the way our process works and the sheer number of evangelical voters there are. What you need first, is 2/3 of both houses of Congress. I doubt you can get 2/3 of EITHER house. You have very high support in very liberal areas of the country. Otherwise, you only have moderate support where you need 2/3 and in some places you have virtually NO support. So the House will likely never forsake the evangelicals and go for a Gay Marriage Act, and the Senate is completely impossible because your liberal states only have 2 votes each, same as everyone else. Look at that popular RED/BLUE map to see how that will work out.
Okay... so now IF you manged to somehow get the Congressional requirement, the Amendment then moves to the ratification process. There, you'll need 2/3 of the states holding specific balloted initiatives to ratify. Gay marriage has struggled to even get majority votes in any ballot initiative, and that is in the most liberal states. In MANY of those states you crow about same-sex marriage now being legal, it was done by courts or legislatures and not by the people. Many of those people are quite pissed off about that. The states where this has been crammed down their throat are not likely going to ratify your Amendment.
This leaves us with the only possibility you have, which is something along the lines of what I have suggested and you have rejected. A Congressional Act to slip the ring off the finger of government and "marriage" where it can be returned intact to the people and churches where it belongs. Such an Act would also be problematic for the same reasons mentioned previously, but it is much more likely than a Gay Marriage Amendment or Act.