Your relationship meter with that sim has to be pretty high before you can woohoo. As far as I know your sim has to be in love with them first (have the love-heart by the relationship bar) before they can do that.
Usually, a lifetime relationship score of 70 (with romantic interactions) will make the sim be in love with the other sim. Just remember, if the other sim doesn't have a lifetime score of 70, he/she isn't in love with your sim, and may reject the woohoo. If you have enough aspiration points, you can buy the lovetub, which will lower relationship requirements for romantic interactions, but they still have to know each other fairly well.
Also, since you can't always tell what your sim's romantic interest's feeling towards you sim is (they may have a higher, or lower Lifetime Rlelationship Score, and a totally different Daily Relationship Score, for example), a good strategy is to look for a big heart floating over the other sim's head. This is a memory of "Falling in Love with a Sim". Be sure that your love interest is in a good mood, or they may reject your sim's advances. And being in love with another sim (your sim being in love with another sim, that is ) opens up a lot more social interactions, most notably (sp?) the option of proposing an engagement (or joining/union). And yes, the hot tub is easiest on your first time if you still aren't sure how the sim-in-question will respond. Hope this helps! Happy Simming , and "woo-hoo-ing"!
Maybe... Not sure, but maybe you need it more basic then that. To actully get the option, you have to have both sims "Relax" in a double bed. Then if they are in love, you will get the option.
good answer elf the hottub is another site that woohoo can take place as well as a changing booth, and now with advent of Nightlife in the front seat of a vehicle and in that photobooth thingy.
