The OP may not 'feel sad' because she is uncomfortable about patriarchal traditions dictating the child has the fathers name, rather than hers......however, patriarchal traditions are still very much at play within this and our society, however progressive we like to think we have become.
So, on one hand women are now free to marry or not, to take the name of the man or not. Indeed, as has been pointed out, they can take the name of their DP if they choose even if not married or he can take her name or they can choose a totally new name if they want . The fact is though,that few people go for the last 2 options and most men DO feel attached to their name and don't want to change it and do feel quite keen for their child to have it too. They have rarely been in a position of being asked to give up or change their name, and historically because of name changing through marriage, their children have had their names.....and losing the 'right' or 'norm' of that unsurprisingly isn't that popular with them. Men are generally not keen to give up their names and like to have their children named after them - even men who very quickly have nothing really to do with the mother and don't have or intend to have much contact with the child. It might be a bit far to go to say that men giving their names to women and children is a sign of possession......but ultimately the historic roots of it, certainly involve this.
Today, we don't see it as this and we like to think we are progressive and have choice. And yes we do have choice, but the fact that so few women give their own name to their child (and even on this thread, for the first 2 pages, it was assumed that the man would give his name - the patriarchal traditions are strongly held to amongst many women as well as men)shows that we are still in the grip of the past values and traditions.
The OP isn't sad that she isn't married to the babies father, nor that she doesn't have his name. She is sad that the baby will have a different name to her,because it makes her feel somehow less connected to that child, perhaps just in the eyes of the world. We know what the world thinks doesn't really matter, but we still do care. I think parents generally want to be seen to be connected strongly to their child. Naming is the key way, the outside world and especially the world who do t know us see that - it is why men want to have their children have their name,and women often want it too and feel uncomfortable without it. In the past when most were married and had the same name, there was no issue, but now that fewer are married, unless going double barrelled, the child can only have the name of one parent, so one isn't connected in the same obvious way,to the outside world. Saying we shouldn't care about this, or it is a patriarchal construct that needs challenging might be true....but it doesn't stop people still feeling sad about it - feelings are feelings.
The complexity of family relationships is clearly shown on this thread - people with names of ex-spouses, previous spouses holding the name of the current partner, people with maiden names and going back to maiden names, step children with all kinds of names. People with 'blended' families want to recognise the blood links to the past often, but to move forward with new families being united. All very tricky, especially when naming is such an emotive issue.