The Scientific answer is, for life to perpetuate itself. So lets put it down to making more life. Short Skirts or Guys going around topless are all part of the erg that each of us has to perpetuate life. We look for in a mate survival characteristics. The Human Body is designed to help perpetuate its form. With Strength and Beauty.
The Philosophic is to better life. Which can be quiet hedonistic as well. I find nothing more exciting that a good debate over an Ale or Something stronger.
Short skirts or topless blokes are not necessary for procreation. They serve their place in our society because the chances for procreation increase with physical beauty for women and traits indicative of physical prowess for blokes (loosely anyways). Thus, without knowing it, dirty tramps run around in mini skirts and blokes rip their shirts off.
However, as I have said, it is not necessary for procreation. All that is needed is for a sperm and egg to meld and form a cytoplast. We have proven this can be done without sex using IVF.
So scientifically speaking, we have the possibility of perpetuating the race without the needs of mini skirts and shirtless wankers. As such, hedonism (once again, loosely) is not necessary anymore. In fact, we will soon (it is only a matter of funding, basically, if we choose it to be) be able to tailor the traits of our offspring to be individually specific. Basically, we can create clones that fulfill the specific needs and roles of a more holistic species.
But then we come to the issue of semi-asexuality, namely that a narrowing of the genetic blueprint variety through specific gene or attribute selection, that works against the very thing that has allowed us to get to the point where we can sit in our loungerooms, behind computers, discussing this very thing in the manner that we currently are. Namely, survival and development through evolution, a process which is based upon random genetic variation and its relationship to a dynamic environment and ecosystem. If we start to clone, we may lose the edge that has got us here in the first place. And so it seems that mini skirts and topless blokes are indeed fundamental aspects of our continuing evolution and survival in a dynamic environment.
But alas, this is not yet the end of the thread. You see, we now also have the means to make our environment largely static, and eliminate much of the dynamic aspect of the environment we live in. In lay terms, we can control and mould our environment. While asexual, or clone reproducing, organisms are unable to survive in a dynamic environment, they in fact thrive in a static environment to which their specific development is well suited.
As such, considering that we have unprecedented control over our environment (once again, further development is only a matter of funding and attitude), our other ability of control over genetic reproduction and development comes back to the fore, and as such mini skirts and topless blokes are selfish attributes that may benefit the individual self-ego but which can work contrary to the larger benefits a society could receive by moving past such pretensiousness.
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But then again, all that means nothing. Everyone wants to look this way or that way, express their individuality to its maximum, so forth and so on. BUT, and this is the clincher, said individuality, sexuality, hedonism etc etc should only be applicable once all communal and holistic aspects have been fully met. Our first priority should always be to our community - friends, family, town, country etc etc. And that is the problem with todays society, that people are more concerned with themselves and their own shallow self-image (amongst other things) than they are with taking care of the more important things - working for the greater good, for the benefit of their country, for the development and progression of the human race. Seriously, if people put towards the sciences the effort they put into their make up or outfits, imagine what we could achieve? Sustainable envoronmental solutions, colonisation of space, serious medical advances that could see us living healthily well past a century, elimination of poverty, so forth and so on.
And, funnily enough, what is the major focus of the braindead idiots today? Australian Idol and Survivor? The latest Gucci items? Cars designed for the very purpose of breaking the speed limit? The Real Cancun? Matching eyeliner to hair colour to what belt you are wearing? The fact that the likes of Cleo magazine even exists is a disgrace on our species. Look at some of the damned Avatars on this site - where is Hawking or Einstein or Zubrin?
I find it funny that a species that has come so far, overcome so many obstacles, and developed so much potential, is being let down by a bunch of people like chloe and her like. But don't you all worry, the time will come soon when their like will become obselete, and they will be left behind to rot in their own filth while the truly elightened laugh at their finely clad, prettily painted, rotting corpses.
Someone has to stir the pot 
) then Earth will fall into insignificance pretty quickly. Because of its frontier nature, it will need to develop am arithmatical development curve in order to survive and prosper. Considering those that will be sent will deliberately be the type of people that will can do this, it will happen. And Earth will continue to stagnate because they will be tied down with massive poverty and social inequality which a bunch of stupid hippies won't let them ignore.
