Marriage - Its Need, Futility & Origin

One of the freakier thoughts that has occurred to me in the past is the possibility that, the only reason for one's existence is procreation and furthering of the species.
That's what all living things do, beyond doubt. They screw!
Every act of life seems to be aimed at this as the final goal. Even aesthetics, mind you. Correlations are deep and intricate, but it should suffice to mention here, that a high aesthetic sense implies a higher level of evolution and therefore a better mate.

Isn't it profoundly depressing to realise that we are all here only because someone screwed someone! I mean, at a fundamental level, do you really believe that, while doing it, that someone and someone were thinking about how a good human being will develop out of their activity?!

Procreation & Marriage
Among human beings, this social function called 'marriage' seems like a vague attempt to cover the fact that at the end of the whole jazz, basically the 2 people concerned are gonna screw away to glory. What other significant difference occurs in their individual lives apart from THIS? (which otherwise cant be achieved in a really good long term friendship with a person of the opposite sex)

So, is marriage some sort of a moral cover up?

I could have said here that I appreciate other animals, for they at least don't cover up. And accept boldly that they need to propagate their species. But they hardly have a choice. And moreover, they don't read this blog.

Human beings, with all their intelligence and years of evolution, are the only species that seem to have an in built control mechanism against 'free and fair sex'. The only biologically and environmentally sound reason for this control that I can think of is, overpopulation. The irony is, that human beings are also the highest in number! what control are we talking here?
I am thus, unable to comprehend, the reasons for the existence of "marriage" as a custom. What does it do? Commitments can happen without it. In fact they do happen without it. Marriages do break. I have known extremely successful live-in relationships, and equally successful marriages. Then where is the differentiator? Where is the need of mandatory marriage (in order to propagate your species)?

Marriage as Celebration
One sane justification of marriage that I tend to agree with is, marriage being a symbol of celebration of commitment, and beginning of companionship! Great.. but why does it then extend into a legal contract? And multiple social obligations? Here is a short hypothetical account of the origin of marriage, with celebration is the premise.

I feel that marriage originated as just an occasion for celebration. Like how you throw a party when you win the 20-20 world cup. It was basically a party, and a symbol of the fact that something worth celebrating has happened in the lives of two people. Soon the trend caught up and these parties became very frequent.
Someday, some smartass who wasn't getting a companion, was too lazy to woo one, and was too eager to wait for the right one, realised that he just had to showcase the symptom (party), and the cause (companionship) will be assumed. So he got another dumbass who wasn't getting a companion and started celebrating. That, I think, was the first day when marriage came into existence!

So, I don't think marriage should be a compulsion. Either social, or moral!
I don't think its necessary for a successful relationship, nor does it guarantee success in any way.

Afterthought
I do realise that these thoughts are too radical. I'd like to tag them as "ahead-of-their-times". But futility if marriage is a question that has failed to lend itself to a convincing answer, at least in my life, so far.