I'm not a staunch opponent, but I can make an argument that demographics do not suggest the inevitability of same sex marriage.
First, the 2010 Census shows that the
two fastest growing religions in America in the past decade are Mormonism and Islam, and both are growing rapidly. As you're probably aware, Mormons and Muslims tend to be some of the most ardent opponents of same sex marriage, and instead believe marriage should be between a man & woman. Or in some of their cases, a man & woman, woman, and woman. This demographic increase works against same sex marriage.
The 2010 Census also show the precipitous decline of America's white population. Despite the
lies that Ted Kennedy and others used to pass Hart-Cellar, the most recent census shows that
whites now represent less than half of all three-year-old children in America. As you're probably aware, it's whites that support gay marriage at a higher percentage than other racial/ethnic groups. While we've seen some
increase in gay marriage support among blacks and Hispanics, their support still trails that of whites. Similarly,
Asians do not support gay marriage as much as whites, and the 2010 Census showed
Asians are the fastest growing race in America (that's good since
they average the highest IQ). As the population numbers of those respective groups increase and whites decrease we could see support for gay marriage stagnate or even trend backward.
Those two demographic shifts, both racial and religious, may work against same sex marriage.