You should also get into the romantic mood a bit before you decide to jump right into woohoo.
You also have the option to get a flirty aura object like the plumbob lamp available in build mode. You can also add the Romantic Aura lot trait which can get both sims feeling flirty rather easy. To get flirty you have a few options, you can get sims to take a steamy shower which works wonders. Feeling flirty will get them both in the mood and may lead to a positive experience including a positive moodlet that goes right along with it.
You should try your best to get your sims to feel flirty so that they can really have a great time when they woohoo. If they are feeling any negative moodlets (i.e., bored, angry, sad, embarrassed, tense) they are either going to reject you or not help your sim have a good time. The first, is make sure that both of your sims are in a good mood before asking too woohoo.
There are a few things you can do to make sure that your sims are having a better time and are less likely to fail. Sometimes your sims are not going to have the best time and may get an embarrassed moodlet from a bad time.
