Let's break this down for ease of the process of understanding.
Age does not define one's maturity, nor does it set an expectation of the level of naivity that a person would display. With all due respect sir, I am stating a point of view, as it is to do on a forum, and again, as you mentioned in relation to somebody else's point of view and definition, the capacity to know something or not know something is not defined by you.
Moving on...
In order to understand why, as Serius enjoys terming, 'friendships', cannot exist based upon attraction or convenience, I suppose it's vital to understand why you think that cannot happen.
Is it because of conformity? You mentioned, Like being friends with your best mates girl, sure she is cool, nice enough but if they break up you know you wont really see her anymore. So, is that out of your own will of 'friendship'? Or is it just because it's 'expected' of you?
Then again I ask, what's the point of a friendship, not even a relationship, if it has no base foundation upon which it stands? '...thats a good way of getting to know some1 right there without the bullshit coverup of a "friendship".' Why are you so keen as to not appreciate the introduction of friends as a thing to value and treasure, but as a throw-a-way, disposable resource that is expendable?
Once again, a trace of conformity. The rules that apply to everyone else, but by some miracle, does not apply to me as to be the reason to my optimism? No. Be realistic. In a society in which the picture has been painted in my mind, it cannot exist at all. It's a world which even in love and relationships, is cut-throat, without moral meaning, and pretty much, destroys one of the last things that end up keeping us from transforming into conformist zombies, in which the rules of society aren't even defined by society itself - it is defined by which the shadow that society walks in.
But moving on from that, lighten up a bit. It's a stressful situation, and we've got to appreciate that without it turning into a flame-fest on opinions.