OH and I were already engaged when I got pregnant, so it's not really much of an issue for me to give our son his dad's surname because if things had worked out differently I'd already have changed my name. I have very little attachment to my own surname, it's a bit of a pain really because people often have trouble pronouncing it (it's quite normal in Scotland, where my family is originally from, but not at all in northern England, where I was brought up, or London, where I am now). I also always have to spell it out, which gets wearing. So I wouldn't saddle my son with it if I don't have to.
Also, the thing about passing the name on - I have a brother who has a son, so my family name is set to carry on fine, whereas OH's isn't because he has sisters who are married or have given their children their partner's name.
To complicate things slightly, OH's surname isn't even the one he had at birth, the one belonging to his biological father, because he and his mum divorced, his mum remarried and OH hasn't seen his natural dad since he was a small child. But he considers the man who brought him up, his step dad, as his real dad, so he has an attachment to his name.
Double-barrelling isn't an option, it'd sound weird! And I don't especially even want to give my son my name as a middle name. There are much nicer names in my family if I wanted to go that route.
I DO think it'll be odd before we're married that my child and I won't share a surname, but it's a temporary situation.