I believe many normal supporters of “gay marriage” are hoping that while it can’t cure/fix them, it will at least get them to stop cruising rest stops and such (living dangerously). Jeffrey Dahmer’s targeting of gays showed how vulnerable they were in a lifestyle where anonymous sex with countless partners is the norm; some hope “gay marriage” will end that.
Which is an impossible hope.
Marriage is the legal framework surrounding a biological imperative--which is the mating behavior of heterosexual humans that exists to guarantee the next generation. Since the entire gay lifestyle is built around the absence of that biological imperative, there is no reason to believe that they will suddenly behave as if they are subject to it because they can now get a legal document.
I've read that gays who have long term relationships continue to engage in the promiscuous lifestyle. It seems to be a characteristic of the culture.