OK, this is going to sound bad, but we got married because I was pregnant and we realised how few rights unmarried dads have over their children.
Neither of us was really 'into' the idea of marriage - dh because he just isn't, me because I'd been married before and it hadn't worked out - but we decided to do it in this very 'calculated' way, simply in order to give dh some legal rights over his children. We'd been together 9 years when we got married and have now been married for nearly 6 years.
Having said that, shortly after we decided to get married and set a date only about 6 weeks later (very small register office do), I lost the baby. I was quite prepared for dh to call the whole thing off, and maybe get married later if/when I got pregnant again (he was the more reluctant of the two of us to go down the marriage route and I knew - or thought I knew - that he was only doing it for the baby). But he insisted that, if anything, it was more important for us to get married after the miscarriage as a way of showing that the baby had been important, but that our love didn't depend solely on the baby. Don't know if that makes sense, but I was very touched (he's not the romantic type...).
I just wanted to tell that story as a way of showing that marriage can be decided on in a very 'cold' business-like way, but can still be a sign of enduring love.
Oh, and I still don't have the same surname as my dh. I went back to my maiden name after my divorce and was not about to change it again! My dds have my surname as a middle name.
(I got pregnant again within days/weeks of the wedding, btw!)