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 don't get why they couldn't just give you some options upon marriage for the names. Like they could have a checkbox for something like "change couple name to.." and then you type in the name you want or a checkbox for "keep last names" in case you DO want both of your sims to keep their last names.
