Start educating the flock about what could happen. First, business as usual but letting them know that when the “plant” comes, as you know they will, then the pastor gets out of the state witnessing of the marriage and witnesses before God alone. Tell them in advance that this will mean they have to be “married” twice if they want to be married in the church and that they should marry first in the church. Then dump the legal responsibilities for the state recognized marriage on the state, where it belongs. It may reduce the number of church weddings but the ones who get married in the church will really want to be there.
I have no problem with that plan. This article points out that the authority of pastors to solemnize government marriages could well be revoked.
I would recommend an intake form with statement of principles to be signed, and among those principles would be heterosexual only. Any unwillingness to sign would automatically mean ‘no wedding’.