Just a showerthought I had, wondered if there was any clear answer to it. Because I'd imagine that it was frowned upon in medieval times and such, and in the 70s it was normal and cool, but I struggle to think at what point in this large timeframe the attitudes towards it (and towards public displays of affection in general) changed.
Are you asking when kissing became publicly acceptable? Tongue kissing? Public kissing? Public tongue kissing? Are you referring to the United States? The "West" more generally? The whole world?
This question seems a bit unanswerable as currently phrased.