I've been with my girlfriend for 13 years. We have a house and two kids. We're not fussed about marriage, but the real reason I haven't proposed is because I have too many friends who'd think they should be my best man and I can't be bothered with the fallout.
@fesshole for god's sake, get a civil partnership or married in a legal way, if you don't and something happens to you, at least in the UK, the law won't protect your family re wills and things. it becomes very messy.