Whether you are married or not you can choose to have a baby and all you have to do is choose the option Woo-Hoo which in other words is getting down and dirty. You pretty much always have a single baby but there is a fun way to have twins. The way you can do that is by doing a woo-hoo and once you see the stomach of the pregnant Sim grow you make her woo-hoo again and presto, you have twins.
You can also achieve that as a cheat - open the cheat box by typing ctrl + shift + c and type in Forcetwins.