Well, a topless woman would not, in and of itself, warrant an NC-17 rating. Not even in a sex scene topless. There have been hundreds of movies with topless women in sexually suggestive situations that have been R rated.

Yes, Bay was trying to avoid an R as he kept telling Scarlett that he wanted a PG-13 and that's the reason she couldn't be topless. That was actually his defense. But if he was trying to get himself a larger audience, the PG-13 certainly didn't help the cause much, now did it?