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… how can a marriage be both valid and sacramental between two parties whose faith choice/denomination baptizes in the Trinitarian form yet denies that marriage is a sacrament and is considered an ordinance only.
What that poster, and you by extension, are saying is that God considers marriage valid and sacramental even when those involved do not believe it to be. In most protestant denoms, there is no concept of sacrament, yet those arguing for validity, opine that just because someone utters the words, "what God has joined together, no man my put asunder, the marriage is a sacrament, thus it can no be “annulled” and subsequent parties must live as brother and sister.
The end game of my argument is that I think Pope Francis and his position about “irregular” situations and thus permission to access the sacraments is spot on given this inanity about “valid” marriages.