... but romance of any kind between a man and woman of different racial backgrounds was not going to happen anywhere else, particularly since that was (for many Americans) the "forbidden" relation dating back to the slave era.
Except there simply was no "romance" involved in "the kiss." Both Kirk and Uhura were visibly fighting it. No "free will," no credit.
Also there have always been inter-racial relationships and marriages throughout the US and world history. Sometimes there were problems because of it, sometimes there weren't. Some folks actually didn't care. Then again, even today some can get hard looks.
Making the kiss the "big, groundbreaking deal" was possible for the reasons posted above, and as long as other series of the period were not going to take that step ...
Except by making it a forced kiss "Star Trek" didn't truly break any ground or take that step either. It's a case of "The Emperor's New Clothes" with most everyone standing around applauding the non-existent splendorous dressings of a naked man.