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
I personally am really glad that it doesn't automatically change their last name. More and more women are choosing to not change their last name, and I'm happy that it's reflected in here. (even though I'm pretty sure that the decision had to do more with laziness than making a social statement. )
