It was first of all not the first. It was at least the seventh. Other (technically) inter-racial kisses had been aired in episodes of Mickey Spillane's Mike Hammer ("Siamese Twinge," 1959), Mission: Impossible ("Elena," 1966), The Man From U.N.C.L.E. ("The Her Majesty's Voice Affair," 1966), I Spy ("The Tiger," 1966), and Movin' With Nancy, a 1967 Nancy Sinatra TV special that aired a year before "Plato's Stepchildren," the Star Trek episode in question.

All but the last of those were so-called 'Asian' kisses, and did not garner negative responce. The kiss between Nancy Sinatra and Sammy Davis, Jr. was an affectionate peck on the cheek, not the fuller* kiss seen on Star Trek.

Both were planned, though the Davis-Sinatra kiss looks spontaneous. That's partly because it was not scripted, while the Star Trek kiss was.

Both were also done in such a way as to confound expected network resistance. NBC feared a backlash from Southern viewers, and insisted on an alternate take without a kiss to be shown in those markets. Shatner and Nichols obliged, but deliberately screwed up every non-kiss take, leaving frustrated network execs with little choice but to keep the kiss in for all markets.** In Sinatra's case, she deliberately shot the scene late in the day so that no other takes could be done.

Negative reaction was expected, but according to Nichols, all mail they received was positive -- including one from an admitted bigot, who wrote:

"I am totally opposed to the mixing of the races. However, any time a red-blooded American boy like Captain Kirk gets a beautiful dame in his arms that looks like Uhura, he ain't gonna fight it."

Several sources I've found claim that the Sinatra-Davis kiss also garnered some negative reactions, but I've yet to find anything else about that.

What makes the Star Trek kiss special is that it was scripted, it was overt, it was the singular subject of the shot, and the show itself was well known, so many people saw it.

* Shatner and Nichols give conflicting accounts of how full it was. Shatner has said the kiss was fake, while Nichols has said it was not.

** Star Trek was already close to cancellation by this point, and execs might have just decided it wasn't worth fighting over.

EDIT: In re the letter quoted above, I can't help wondering what the fellow thought about this kiss. Or this one. Or this one.

Note also that we've never heard boo about this little peck.