I spend an inordinate amount of time explaining to people who feel like they have the right to ask, why I'm not married. Not not married yet, not married.
So I rather enjoyed this article from Jezebel (oh Jezebel, if only you were a little more consistently awesome, rather than so uneven): Ten very good reasons why you're not married.
My particular good reasons are summarised by 'You are focused on your career' and 'You have got a life and friends you are happy with'. And possibly a little of 'You have got standards', but my standard is usually 'hasn't been offered a good enough argument by [Interesting Hypothetical Guy] that changing the way I live my life is worth it'. And it would have to be a super argument. A really good one. With powerpoint slides and maybe a song.