just make sure you have two sims who are partners.
you get this buy doing actions in one house.
first talk then do romantic till the status changes
when you are partners so you have a relationship you get automatically the option to do WhooHoo with the other sim.
to make sure they stay together i made them live toghether first.
then wait till it is friday and click one sim to interact with the other and choose the whoohoo option.
your quest should be done after 5 minutes