The bible doesn't even say anything about gay marriage. And neither homosexuality or gay marriage is incompatible in any way with Christianity. Not all Christians, not even a majority anymore, go to church and "listen to their pastor attack gay marriage." Religion is always an evolving thing and people are certainly free to support or oppose whatever they want within that, but when they do choose this battle, it does promote and encourage misunderstandings and ignorant prejudice against Christianity and religion in general.
The Christian case for gay marriage - Los Angeles Times