Get another Sim's "Romantic Interest" status up, high to full will do, by doing friendly and romantic interactions with them. Soon enough the romantic option "Woo Hoo" will be available click it and watch the magic happen.
Alternatively you can click on your bed and choose the "Relax" option. Then pick the Sim you are romantically interested in and choose Make "Woo Hoo". This way of doing it is helpful because you can make "Woo Hoo" sooner than the romantic option is available. You will need the other Sim in the same house though.
This is made a lot easier if you start the game with a male and female Sim.