I Don't.

My mother and I are two very different people and, as I have discussed here before, I suspect she's always been a little disappointed that she didn't get the best friend she wanted in me, but there is one issue on which we agree whole-heartedly: marriage or, more specifically, the determination to avoid it.  We came to this decision for different, but equally compelling reasons.
For mum, it was experience.  She married my father in 1968, having had to practically scream her acceptance of his proposal to be heard above the operatic din of friends and relatives trying to prevent her from making the biggest mistake of her life. 
She loved him. 
She knew him better than any of them. 
She could change him. 
Her honeymoon was spent in a sparsely furnished one bedroom flat which was extremely ecologically friendly for the time - thanks to the power being cut off the month before.  She hung in through seven years of utility restrictions, eviction notices, soup dinners and Dad's virtuosic ability to quite literally charm the pants off any female within a ten mile radius (including her bridesmaid and best friend across the street), until he finally exhausted her patience by selling their house while she was in hospital to cover personal debts.  The only man she has even considered marrying since then was engaged to her for thirteen years before the realisation finally hit that he wasn't planning on buying a partner for her lovely diamond solitaire ring any time soon.  Understandably, all of this has left her somewhat sceptical of the concept of wedded bliss.
I have never been married.  The closest I ever came was living with the father of my son in a de-facto relationship for five years, the demise of which, I can admit now, was as much my fault as it was his.  My aversion to matrimony, therefore, can not be attributed to experience.  My motivation for staying single, despite exasperated argument against it by my best friend who, bless her, still believes in happily ever after, is a selfish one.  I am what I am, and I ain't gonna change.  I have been living on my own for twelve years, and have consequently developed a lifestyle.  It isn't always a happy one, but it is a comfortable one.  I keep house appallingly, I shop with gay abandon, I eat like a condemned man, and I raise my son according to the unfortunately named 'Free Range' philosophy.  These are just some of the things on which I am unwilling to compromise.  From what I've observed of other people's marriages, the words 'I will tweak my life until it comfortably blends with his/hers' should be included in every honest couple's wedding vows.  
I suppose chucking the idea of marriage over mid-week chocolate feasts, relaxed bed times and a mild eBay addiction seems trivial, but these little things and many others make me who I am.  Were I to get myself into a situation where I allowed someone to chip away at my little habits and routines, I'd be losing myself, piece by piece, until it wasn't even my life I was sharing anymore, and the man laying next to me would eventually wake up wondering where the woman he married had gotten to.  I want love, but I'm not sacrificing myself to keep it.  I am a slovenly, gluttonous, stuff-addicted hippy.
I am me. 
If you can't see yourself standing with me before a full length mirror without wanting to flip it over, don't bother taking the tux off the hanger.