This is a very interesting yet weird discussion, so emotionally charged from both sides.
Let's try to unpack this and remove some layers of complexity around morality or the right to wear what we want.
here is a clear example
***warning if you click on this link you can never unsee this image and it may forever haunt you***
https://stevehollier.files.wordpress.com/2010/10/borat-mankini-very-nice.jpg
Now, I think we all agree Mr Borat up there is not appropriately dressed to enter the Lounge or be seen by a member of the human race anywhere essentially, we would also agree that the way he dresses would affect others, and at the very minimum make a significant subset of people highly uncomfortable.
Now that we agree it's not black and white and anyone should be allowed to wear anything they want as long as it's clean, the question becomes more about what is considered socially acceptable to wear in public places in order to avoid making at least the majority of people around you uncomfortable.
You will find this varies from place to place, for example while a bikini would be appropriate for a beach, it would make people uncomfortable at a restaurant or extremely uncomfortable in some more conservative countries. a lounge is no different from a restaurant, hotel lobby or other such public place, and one should wear clothes that are socially appropriate for that place.
The reality is different people have different sensitivities about which parts of the human body should be exposed to make one uncomfortable, and the spectrum varies significantly from person to person.
Knowing you will meet people of all cultures, backgrounds and ages in the lounge, it would be polite and considerate to dress to a standard that is less likely to offend, that standard is often referred to smart casual.
This has nothing to do with socio economic standards, expected behaviour, perceived social class, narrow mindedness or any other labels you wish to stick to it, it's about politeness, respect and general etiquette.
So, in conclusion, please be respectful to your fellow travellers who may have different sensitivities and cultural backgrounds to your own (this applies to older generation Australians and more conservative individuals just as much), it takes 5 minutes to change clothes when you arrive at the beach on the other side, is that really too much effort to be polite?