Opponents of gay marriage are largely motivated by their firm belief that such unions are displeasing to God and that only divinely-approved marriages should be permitted. Unions which offend God should not be legal. In the past, this meant that conservative Christians opposed interracial marriages because it was God's will that the races be separate. Today it means that conservative Christians oppose gay marriages because it's God's will that marriage be between one man and one woman. Where will it end, though?
How long will it be before they start fighting for other kinds of limits on civil marriages based upon their beliefs about what their god wants? For example, should atheists be allowed to marry? Godless people cannot be united in "Holy Matrimony" and their marriages cannot in any way be based on the will of any gods. From a faith-based perspective, this makes godless couples just as much out-laws as gay couples today, or interracial couples in the past.
If Christian Nationalists who follow dominion theology get their way and enact laws which ban all sexual activity which is contrary to the will of their god, it's hard to see why godless couples wouldn't be included in the bans somehow. On the other hand, if a person finds it absurd to even consider banning marriage or sexual relations between godless people, then the door is opened to denying the validity of other faith-based restrictions.
If religion isn't a good reason to prevent atheists from getting married, how can it be a good reason to prevent gays from getting married? Homophobia in America is fueled almost entirely by traditional, orthodox Christian doctrine. What this means is that gay liberation won't occur without undermining traditional Christianity and religion-fueled prejudice.