Legally prohibiting women from going topless seems unequal and wrong, but from a cultural standpoint I think it often makes sense for women to wear a top. In most US states going topless isn't illegal, it's just not culturally done.
As much as some try to argue that breasts aren't sexual, women's breasts are secondary sex characteristics and erogenous zones. Nipples can become erect with arousal (or randomly) just like parts of mens' anatomy that our culture requires covering. And at least 25% of women can be brought to orgasm from nipple stimulation alone; it's nowhere near that number for men.
Even in places where going topless is culturally acceptable, there may be common sense limits. It's one thing for small-to-medium breasted women, say up to about 34C, to go topless. But large breasts bouncing around topless and unsupported can be uncomfortable for the owner and distracting for others, so maybe the top should stay on. Just because something is (and should be) legal doesn't mean it's the right choice. Obese men (should) know not to wear speedos to the beach. Certain female body types don't really go with bikinis. And the same with public toplessness; what is ok for some just doesn't make sense for others.