I think it helps when the relationship is already established (i.e. Will and Elizabeth have known each other for years and are obviously in love, but can't marry because of class differences)

Good point. If Will had just walked in to deliver his master's sword and had fallen in love with the Governor's beautiful daughter on the spot, I don't think the relationship would have been the same.

(Though, thinking about it, Robin and Marian are basically enemies-to-lovers in "The Adventures of Robin Hood"...)