I misunderstood; from the way you phrased it, I thought you were asserting that such a thing had already taken place.
For the record, I don't foresee atheists being able to force churches of which they are (presumably) not a member to perform weddings for them.
The way I phrased it is “will” and not “are”.
Couple the hostility seen from athiests with recent government and foolish people within the church, it is possible.