I think the homosexual movement is winning the argument over homosexual marriage, as well as homosexuality in general, in the United States. I expect acceptance of both homosexuality and homosexual marriage to increase in the coming years.
I just saw Don Schweitzer, a lawyer who opposes homosexual marriage, interviewed on "The O'Reilly Factor". Schweitzer's poor performance is typical for opponents of homosexuality. Bill O'Reilly began the interview by dismissing religious arguments as irrelevant. Schweitzer didn't object, and he went on to attempt, ineffectively, to make a non-religious case against homosexual marriage.
The best arguments against homosexuality and homosexual marriage are Biblical. Instead of accepting the ridiculous non-religious framing of the argument (God owns and is relevant to all of life, not just some portions of it), opponents of homosexuality should keep the Biblical argument against homosexuality at the forefront and expose the fact that their opponents are incompetent to address it. There's some merit in also pointing out some of the other problems with the pro-homosexual movement, but the Biblical argument should be at the forefront, not left aside as irrelevant. Have you noticed that the more opponents of homosexuality accept the dismissal of religious arguments as irrelevant, the more irrelevant the opponents of homosexuality become?