What an interesting question.
I recently entered the building contest with a dormitory. I originally planned to plant a tree house in the garden (as one of several potential places for woohoo as I didn't include any double beds) however, in play test mode, there were no options for woohoo. As I'd never tried this before, I just figured that maybe I'd got it wrong about being able to woohoo there and removed the tree.
However, in my current game, I've just added a treehouse for the youngster and the option to woohoo has come up for his parents.
How curious.