Let me have a go at explaining this in a way that I hope won't make me sound like a bitter and twisted spinster (I'm not by the way, but I'm sure some people will find ways of misinterpreting this).
Among my married/long-term cohabiting friends, there are some who have marvellous relationships. They are friends, they support each other, they have equal partnerships where they treat one another with consideration and respect. Obviously, it would be lovely to be in a relationship like that (and I realise that a lot of hard work goes into maintaining such a relationship - not taking one another for granted, continuing to communicate even when knackered out by a teething baby etc. etc). I just haven't been lucky enough to meet a man with whom I think I could have such a relationship.
I also have friends who desperately wanted to be married (mostly female). On the whole, they have paid a huge price for their married status - their careers have gone down the pan, their husbands don't do their fair share of the housework, they are taken for granted and put in second place. I would not change places with them for all the tea in China, not even when I'm trying to cope on my own with a sick child while running a temperature of 40 myself!
As for my single life, I have a delightful child, lots of friends, a job I find interesting and feel is socially useful, snatched moments of a life beyond motherhood (playing music with friends occasionally when DS is in bed).
So if I was lucky enough to have a partner of the right sort I could imagine feeling very happy, blessed and fortunate. But I would not feel proud of being a wife. Proud of working through the difficult patches in a long term relationship and emerging from the other side stronger, I can see that makes sense. But proud of the attached state as an end in itself? No, I just don't get that.
And as for titles, if I did get married, I'd keep my own name, stay Ms (or indeed Dr - and contrary to the comments up the thread, I am bloody proud of being Dr Lurcio because I worked my arse off for it). But if another woman chooses to be Mrs Husbands-name, well, that's her choice. A bit weird, but so's eating marmite and to each their own.
Anyway, thought I'd have a go at explaining that it's possible to be single and happy, yet not embittered, and at the same time admire those who make good marriages work and feel they've got something worth having. And point out that, viewed from the outside, not all marriages come across as a good place to be.