In an OPLA verse version of the “Zoro and Sanji met and started dating before the Straw Hats formed” premise, the reveal of their relationship would go like—
they behave totally as they do in canon up to when Sanji brings the Straw Hats their drink orders.
then Sanji intentionally starts shit with Zoro (“You know, we do also serve food here.”). they start bickering (there are reaction shots from a wary Nami, a raptly interested Luffy, and an increasingly alarmed Usopp) until Zoro legitimately draws Wado and takes a swing at Sanji.
Usopp and Luffy both start out of their seats but Sanji blocks Zoro’s swing easily. there’s a beat of total silence—even some of the nearby tables are watching, stunned—and then both Zoro and Sanji crack up. Zoro sheathes Wado and just fuckign. does the scoop-up-and-twirl-around-into-a-big-ol’-smooch maneuver on a grinning Sanji right there in the middle of the dinner service (reaction shot of the other Straw Hats, looking various degrees of flabbergasted).