Same sex marriage is not wrong. Giving it legitimacy and normalization is wrong.
Why.
Because it is a poison to the larger societal structure in which it exists. Homosexuality itself is benign. It does not harm, nor does it help. When it is accepted as normal behavior by the host culture in which it exists, it becomes destructive to the whole.
Mostly because acceptance of the normalcy of same sex relationships comes along in a package of degeneracy. Perhaps it could not exist without a pool of depravity to feed it. Once a culture gets to normalizing same sex relationships, it is already very sick. Rather than look to getting stronger, the culture embraces the infection imagining they are getting stronger.