Usually the root of the argument boils down to your exposed breasts will titillate men and cause them to act out of the lust in their heart. If you dig, you’ll often find statement to that effect made by the legislators or lobbyists responsible for laws against exposure of breasts.

It contrasts with professed love of freedom, but humans are always creatures of paradox.

Then we should also ban advertising, because it causes people to act out of envy in their heart.

Doesn't have to be "scientificaly substantiated" harm.

Killing someone is not a scientifically substantiated harm either. Of course we harm the other person's health (that's objective), but that in itself is only bad in the sense that "life is good" and "we should respect life / not kill". And that is not a scientific argument but a moral one. The universe doesn't care either way.

What does "scientifically substantiated" mean to you?

The same thing anybody would understand by the term. Did something gave you a different impression?

That there's a scientific proof that harm is being done by an action.

But harm is a moral term, and science is not about moral judgements. It can help inform them, but the guidelines for what is "harm" or "good" come from morality ultimately.

E.g. what is "good for society" is based on a value judgement / decision by said society based on its goals, what it wants to conserve or change, etc.

Science can tell you that X technique is more productive for example. But whether "more productivity" is desired is a matter of social values/priorities, not a scientific matter.

To be honest that topic people can debate. Men are allowed to walk around without a shirt. Women breastfeed in public pretty frequently.

There is however a long history and tradition of humans wearing clothing to cover their genitals. Even very primitive societies have this norm, and it has pretty obvious reasons behind it.