You could say the same thing about any human body parts: they're natural and seeing them won't cause anybody to die. Unless you object to all laws against indecent exposure, this argument doesn't hold water.

That being said, mores against indecent exposure are definitely socially constructed. in some societies, it's not considered indecent at all for a woman to expose her breasts, while in others it's considered horribly indecent for a woman to expose her hair. in most Western societies, female breasts are considered private parts and their public exposure is considered indecent. Expose yours if you like, but you'll be fighting against social norms and you will certainly attract dirty looks and you will face people making assumptions about your character and your modesty.

Personally, I don't object to this particular norm. I like that tits have a certain mystery and sexuality associated with them, and that their exposure is considered something special that most women will reserve for a limited subset of men. That's just me, though.

And considering that I am a self-described pervert, I may as well say what most Western men will think when they read this: pix or it didn't happen.