You have to make them fall in love first.
You can do this by making them flirt and then making your way up to makeout ( which I believe is the highest level you can go with just you and the person (meaning no items involved).
After you get this far you can makeout on the couch, propose engagement and marriage(you do not have to be married to woohoo.) and you can also, at that point, woohoo in beds, photo booths, changing stalls, hottubs, cars (if you have nightlife) and elevators (if you have Open for Business.)
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You can get married but after that no (well if you can i haven't been able to do so and I've beat the game and had it for months) you can't have wahoo eaither
You can not make a Sim anorexic.
You cannot make a sim nude in Sims Castaway.
If your Two sims are in Romance level (You know this if your sims can Make out) get both of your sims to relax in Bed by clicking on the bed and press Relax. Make sure both of your sims to do it. Then click on the sims you want to Woo Hoo with and press Woo Hoo or Try to have a Baby
You can't.