In the case of seatbelts, you have a big change in human tradition. There are very few societies that have ever required all people to wear seatbelts all the time. I can't think of any examples. A small handful of modern societies require it for specific subgroups, but that's it. Generally, extraordinary claims require extraordinary evidence. In the case of seatbelts, I want to see very strong evidence that seatbelts are very necessary, and other approaches wouldn't have similar or better results with lower change to human tradition. So far, I haven't seen that at all. The scientific evidence that seatbelts are effective or necessary is weak. Alternate approaches (eg. improved driver training) may be just as effective. The examples you cite have roots in human tradition, and have been established for many years, mostly with strong evidence.

You still have freedom loving individuals in places like New Hampshire who embrace beltless freedom, or even more stupidly riding motorcycles without helmets.

At least in those cases, that “freedom” usually kills only them when things go wrong.

Well, seatbelts aren't the be-all, end-all, either... And people have indeed complained about them when they were mandated. So, it's not like "one could say the same thing about seatbelts, so look how absurd it is". Someone else might easily reply "my life, my choice" about wearing one (same as for drinking, smoking, over-eating, etc).

But even so, we can consider seatbelts as a necessary wearable/accessory for car use, which is also a recent development (cars and driving). Masks are different in that they affect people going on about doing things people did for millenia without requiring a mask.

What harm will befall people who see my breasts and what strong scientific evidence can you provide to substantiate that theoretical harm?

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.

United States, 1918.

Instead of projecting arrogant puffery, why don’t you think about the potential consequences of prioritizing your comfort and self aggrandizement to people whose lives are put in danger.

I have a neighbor who unknowingly was infected early on and likely infected several of her elderly clients, most of whom are now dead. She is devastated... think about how you will feel if you did the same thing, without the innocence of being ignorant.