I believe gay marriage supporters opposed this because it leaves them without a fight.
‘Zackly.
Without the coercive power of the state they are helpless against societal disapproval.
Moreover, the LGBT crowd are always mentioned as such as if they are in lockstep solidarity. They are not, except in made-for-teevee-protests and political pandering. As more than one person has mentioned here or elsewhere over the years, most people aren’t as bothered about lesbians. Gay men may have a hormone imbalance contributing to or causing their condition but they still have testosterone which causes them to act aggressively, if effeminately at times. Boil down this conflict and accurately identify the players and the plot and you find that the battle, as usual, is about sodomy. Gay men are ashamed of the unnatural practice and of the societal disapproval that will never vanish. The shame turns to rage and the rage turns to taking ‘revenge’ for their own actions! Nonsensical but there it is.
This rage manifests itself in the legal challenges, the militancy and the pseudointellectual columns, magazines, etc. that all attempt to put a normal face on the abnormal.
When they realized that they could take and use otherwise sleepy county clerks’ offices as the instrument of their triumph, they rejoiced as any coward driven mad by societal disapproval would.
The destruction of the institution of marriage is their main goal
Goal achieved