That doesn't necessarily mean the vote will be a victory for gay marriage opponents, however. The emergence of same-sex marriage as a political issue has benefitted Republicans and hurt Democrats for two reasons. First, it has motivated conservative voters to get to the polls. Secondly, it has served as a wedge issue -- many people who might be inclined to support Democrats also oppose gay marriage.
Importantly, to continue to serve the second function, gay marriage bans can't pass with 51% of the vote in Republican states. Instead, they have to outperform the base Republican vote in the state. A pretty good estimate of Texas' base Republican support is 58% to 61%. Gov. Rick Perry received 58% in 2002 and President Bush received 61% last year.
I would say, therefore, that if the ban takes 65% of the vote or more it is a victory for social conservatives. If it receives much less than that, it will be a moral victory for gay marriage supporters. Turnout dynamics are so strange in off-year elections, however, that it wouldn't make sense to draw any overly sweeping conclusions from this vote.
(By the way, last year the 11 gay marriage bans on the ballot outperformed Bush's totals in those states by an average of 12.27%. The two extremes were Mississippi, where the ban outperformed Bush by 27% and Utah, where it actually took 6% less of the vote than he received.)
