What last name do you give the babies of unmarried sims?

You missed an option for your voting. I do the same as @luimistelija and give either the mother's or the father's last name. I base it on to whom I am going to give main caregiving responsibility for the child more than anything else though. If mother and father live in the same household but are not married, I usually go with the father's last name if it is a boy and the mother's if it is a girl. I just hate hyphenated names, they are too long... lol