They think legally allowing them to marry will truly make them happy. They will be found wanting.
What they truly desire is to shout down that internal voice that’s saying “this is wrong” (Rom 2:14-15).
The subconsciously think that if there are no external voices implying the wrongness, or more to the point, that if all external voices are shouting “gay is OK!”,
then their internal voice will be silenced.
It has to do with access to divorce courts, the need for which makes tragic sense since often one member of the socially necessary pact to bring children into the world and raise them fails miserably in performing the responsibilities to which he or she has agreed.
There is never ANY need for two men performing acts that can never bring about children to have access to divorce courts.
But the "gay marriage" debate is never spelled out as such. It presents the image of it being a ceremony with a cake and a minister and tux & such and there is no law keeping gays from doing this now.