Everyone. Gay marriage isn't just a morality issue, its also a practical one. Exactly what benefit does having two gay people married bring? Little to none. Exactly what benefit does having two straight people married bring? Kids(the future).
So here's the deal. We should change the law to unincentivize marriage alone, and start giving tax reliefs to marriages that have X amount of children, the more children the more tax relief. Then legalize gay marriage, then if gay people want to adopt, they are helping out society and get the exact same treatment as their counterparts who have the same amount of children. Wham bam thank you mam, equality that is also practical.