YTA. The fact that you never expected to have to negotiate this just means that you've been benefitting from patriarchal male privilege. You're TA because you're complaining about having to give up that privilege. Why don't you take her name, so the child's name will be the same as yours? Or talk with her about combining your last names (not necessarily with a hyphen; you can also meld portions of them such as combining Smith and Jones into Smones or Jith - my husband and I chose this option) or just picking an entirely new name that pleases both of you. There are many reasonable options to try to negotiate for, but assuming that your children should get your last name and not hers just because most people you know do things that way is sexist and not reasonable.