From Fanlore, the fan run wiki about the history of media fandom:

“The first public discussion of K/S may have taken place at SeKWester*Con, Too  [1977]  in the form of a panel called Kirk and Spock: Do They or Don’t They?. ^[9]  

It was, by all accounts, the most well attended event at the convention and quite boisterous. 

On the panel were Leslie Fish, Gerry Downes, and Gayle F., Carol Frisbie, editor of Thrust, informed the audience that Gene Roddenberry knew about K/S and had told them that, “as long as it stays underground, and away from the mothers of America,” he had no problem with it.

Parts of the panel went unintentionally public when a problem with the hotel sound system inadvertently projected the voice of Connie Faddis reading aloud from an explicit story into the hallway.  

And while the panel was controversial and generated loud objections (see the LOCs in issues #12 and #14 of Menagerie), the convention organizers pointed out afterwards in Menagerie that when the panel was announced in the convention progress report: “Ten people volunteered to be ‘pro’ the K/S relationship; no one volunteered to be 'con.’ We had to draft the opposition.” (Panelist Theresa H. revealed that she and other “anti”-K/S fans had felt intimidated by the “pro” side and that many of the fans she spoke to had been afraid to be “emotionally involved.”)

Also see: The SekWester*Con Porn Debate and The Sound and the Fury: The First Panel Discussion About K/S.

For a broader discussion of these issues see: Slash Controversies and History of K/S Fandom.