They might also consider it a sacrament when twice-married-and-divorced Sally weds thrice-married-and-divorced George. However, if Sally and George wanted to become Catholic, their marriage would not be recognized until and unless a tribunal could determine that their previous marriages were all invalid. Their ceremony in the church down the street didn’t create a valid marriage.
Similarly, if Jim and Tom have a ceremony in the church down the street, it does not create a valid marriage. In order for a marriage to be sacramental it must be a) valid and b) between two baptized Christians.