We had lots of debates about this. We are not married and know we never will be. But I also know that had we ever married I would have kept my name anyway.
I suggested ds have my surname for practical reasons: it is higher up the alphabet, there is only one possible spelling of it, it is slightly less unusual than his.
The argument that swung it for me in the end: as a teacher, when I see a mother with a different surname to her child I think nothing of it, it is very common. When I see a father with a different surname to his child, I assume he is not the child's father. My assumption is, I'm sure, a terrible prejudice, but it is a common prejudice, and not one that it would be fair to inflict on such a doting father!
After this realisation I agreed a little reluctantly to ds having dp's surname (middle name or double barrel just didn't work for us). However, when we chose first names, we chose a male and female name very closely related to my first and middle names. So then I was more than happy for him to have dp's surname - he had something of me too! In fact, many people have commented on the close relationship of ds's name to mine and how nice that is!
Sorry, I know I'm going on, but one more thing. I always answer to Mrs dp. I don't correct them unless it is going to be a longer term relationship. The only thing I am paranoid about is ds being involved in an accident, and someone ringing work to ask for Mrs dp and not finding me. So there are signs above all receptionists desks saying ds's surname!