> EVERYONE does this stuff. Why hide it?
Because it is good manners to do so. Good manners exist for a good reason: we have to constantly be around other people and behaving well in public eliminates unnecessary interpersonal conflict. Good manners help you put your best foot forward, while poor taste, crudeness, and indecency will always lower other people's opinion of you and damage your reputation, your job prospects, and your chances of getting what you want when dealing with others.
Good manners have nothing to do specifically with hiding sexuality, boobs, or the fact that people masturbate. We do not expose these in public because they are unflattering, not because they are shameful. Society has reasonably well defined expectations of what is and is not acceptable to do in public. These expectations are there to help you by telling you how to behave in order to be perceived as a person worth dealing with.
Good manners also help other people to go through their day without having to deal with things they find offensive, ugly, or just in plain bad taste. People who spend their days surrounded by what they like and enjoy are happier, more productive, and easier to deal with.
Society's standard of good manners does change with time, but the reason for having them does not. As long as you always try to show your best side in public, you'll be ok. The problem with the situation in the article, Miley Cyrus, or whoever is currently getting on your nerves, is that these people do not try. Not only do they fail to show their best, but they flaunt their worst - this is the real tragedy and a sign of the decline of our civilization.