First of all, why all the fuss over the name? YOU are the mother, YOU are the one who will be taking care of your baby full time, YOU are the one who will do all of the day to day care, YOU will wipe the tears, YOU will change the diapers, YOU will take care of everything, so if you want the baby to have your last name, I say do it. I would.
If something as minor as the child's last name will effect the father's relationship with the child, then there wouldn't have been much of one in the first place. Your child will still get to be siblings with his other children, so what if the last name is different? It will make things easier on you and right now, that is what matters.
As for others saying that if you get married you will have a different name than your baby, that isn't true. No where does it say that when you get married a woman HAS to change her name and no where does it say that if you get married that your new husband couldn't take your last name. Better yet, no where does it say that you have to be married.
If you feel that the father deserves some sort of distinction, although it doesn't really sound like he does, then I would give your child his last name as a middle name.