The argument is this. The point of same-sex marriage is to make SSM 'equal' to traditional marriage. But if some churches don't accept it, it's not equal: heterosexual marriage will available everywhere, but SSM only in certain, restricted places.
Compare the argument of people campaigning for women bishops in the Church of England. If 'traditionalist' parishes can opt out of being governed by women bishops, they aren't bishops in the full sense. No one is allowed to opt out from male bishops: there is an inequality here. Because of the attitude of their opponents, they would be deprived, under some of the proffered compromise schemes, of true episcopal authority.
It is true. In both cases it is true: the hold-outs, even if very few in number, aim to prevent the gay couple or the bishopess from being interchangeable with a mixed-sex couple or a male bishop. If the hold-outs get even a 'room to breath' concession, they have in a sense won, because they have stopped the innovation being made completely normal. The innovation remains something only allowed in certain places.
There is no room for toleration here, friends. This is a fight to the death.