As with previous Sims games in the franchise your Sims Woohoo to produce new life. To do this you must first raise your relationship with another Sim to a romantic level using the 'Flirt' options in the conversation menus. When you reach Level 8 you need to buy a Object Level 2 bed and place it in a bedroom. Then just interact with it with one of your Sims and select the 'Woohoo' option.