In South Africa not all priests of any denomination are automatically marriage officers. When my daughter got married, Father J, who was to preside at the wedding was not a marriage officer. Accordingly, Father N, who was a marriage officer would be present at the practise on the Thursday evening, at this time daughter and future son-in-law would exchange vows in terms of legal requirements and would sign the legal documents that would make them married in the eyes of the state. However Father J was most emphatic that they were NOT to consider themselves married at this time and were NOT to consider any physical relations until after their wedding on the Saturday afternoon.
Apart from the evidence quoted elsewhere in this thread, this is sufficient for me to understand that they would indeed have been gravely sinning had they assumed that the civil marriage, to comply with the laws of the land, meant they were married in the sight of God. (As it turned out, Father J was called to a lunch-time meeting with the Bishop on that Saturday, and, as he was afraid he might not make it back in time for the wedding, Father N took the whole thing over, so the legal and church wedding took place at the same time.)
I can’t help wondering why, TheAdvocate, are you seeking a “more nuanced answer” to a very clearly answered question?