Cactus - I am NOT flying the flag for marriage here, just saying that it's interesting how many men are very firm about being traditional for the bit that suits them
Of course, that makes the assumption that it is the man who doesn't want to get married, not the woman... 
(Which always amuses me as it is the default opinion of most people when considering a couple who are unmarried)
It is possible to have a considered decision about these things. "What girls name do you like? What boys name do you like? What surname do you like?"
It's just that when the man's surname is chosen the same default opinion, that it was decided on because of tradition, is reached by many people.
Personally, we had a discussion about all of DD's names before she was born. We'll have the same discussion about all of any future DC's names before they are born. I would quite like any future DC to have my surname; one of each. DP agrees with this too. Though it will depend on what first name we chose and what we think sounds better vanity alert
The thing is there's no right or wrong and you can only go with your gut feeling. I do understand the arguments about women disliking having a different surname from their children if they split up with their partner. But that ties in with whether you are bothered in the first place about sharing a name.
To me, DD's name is her name. As someone said up thread, to me a name really is just a name, it doesn't matter. We might have considered giving DD a completely different surname to either of us. Its part of her unique identity and not, in my personal feelings, overly important how much it reflects the identity of either of her parents.
(But that's just me waffling, and no one should ever be railroaded into giving their child a surname they're not happy with!)