Not sure about that. Also I am predicting people voting Lady and the Tramp only due to the fact that it's older and not because it's more widespread. Upside down kiss easily is as much as widespread(iconic) as the spaghetti kiss imo.
I think both are iconic but I'll have to say lady and the tramp as it's probably one of the most parodied romantic moments I've ever seen. The upside down kiss is iconic buts its so inherently spider man I rarely see it parodied in other series or referenced as often