Unlike Sims 2, getting your sims to whoohoo in Sims 3 takes a bit more socializing. If you note in the upper left corner of the screen when they're socializing, it tells you what one sim thinks of the other, example Jeff thinks Stacy is being flirty. You need to continue to have romantic socials between them until you get the message that the sims think they are extremely irresistible. When you check the romantic socials, the woohoo option should show. Also their mood should be high, if they're exhausted or starving, then it'll ruin the 'mood'. Hope this helped.
