Who decides, or enforces, that it is a man and a woman?
I think the point is that the state is going to use legal power to force churches to perform marriage ceremonies for couples that are not a man and a woman.
It would be one thing if the state just wanted to force its own registry offices to do this, in which case it would be a question of conscience for employees who do not agree on moral grounds, but would not exert power on the churches themselves. The state and the gay lobby are not satisfied with this, however.