Last name didn't change after getting married.
I moved two sims in together and when they got married, the female still has the same last name she did before they got married, and their child has her last name.
Forum Discussion
Perhaps they should have a pop-up when sims get married or have children asking the player what their first and last name should be? Like how it asks what a baby's first name should be. That way people can do as they please. Call your kids Willabilly Flumpypants if it so pleases you. :)
