Not acceptable.
It's not even acceptable for men to wander around towns and streets without a shirt on.
The only time it is acceptable for a man is at the beach, or at a swimming pool, because they are swimming.
And the distinction here is that the breasts of men aren't loveual in nature, whereas a woman's is.
A woman's should be covered up at all times in public (bar specific locations, like nudist beaches and naturist retreats), and a man's should be covered up at all times bar the above.
It's public decency is all.
You don't go around showing off your genitals, so you don't go showing off your breasts, particularly in public where there can be children about.