Again, a two-second bikini shot...
Look at it this way.
The half-naked shot of Kirk in the film serves to reinforce the idea that Kirk is not taking things entirely too seriously still.
The half-naked shot of Carol Marcus in the film serves to reinforce the idea that...Kirk still has some issues.
One could argue that, for equality's sake, if you're going to have a half-naked scene of a male character that at least has some minor point, then the equivalent half-naked scene of the female character should at least be relevant to the character development or SOMETHING regarding the female character.
You're right, it's only a two-second scene, which makes it a not particularly strong example of what people find objectionable about it, but it is still an example nonetheless.