Well, they were being forced into it. There had been no sexual tension between the characters prior to that, so it would have been at best annoying. Note that even Chapel wasn't happy about it, and she did, in story, have the hots for Spock.

In real life, story is they intentionally kept screwing it up to force more retakes. Hey, I was "forced" to kiss Nichelle Nichols in 1968, I'd be using excuses to keep doing it as many times as I could too.