In all archival honesty, "it" was not exactly television's first example of an interracial smooch; several others preceded it, but those instances have been lost to time and space.
Here's the breakdown....
The Platonians force some match-ups: While Kirk and Uhura dance into position, Chapel and Spock are seated and ready to interact. "Interspecies Kiss"?
The lull before the storm.
"Plato's Stepchildren" is not a good episode of Star Trek. "Bad"? As a friend said to me years ago: "It's like they had a cool set and came up with a story to use it."