The Sim gains a the dry spell buff when they have not had WooHoo or kissed a Sim in a while. To cure the dry spell moodlet you have to WooHoo with or kiss another Sim. Kissing will keep the dry spell buff at bay, but needs to be applied far more often than WooHoo. WooHoo also comes with a decent positive buff that last 3 hours, to be replaced by a lesser positive buff that lasts a further 3 hours.
Two Licentious Sims do not need a 'Romantic Interest' relationship to WooHoo, 'friends' apparently comes 'with benefits'. The WooHoo option will not appear in normal/romantic dialogue, but the tent allows "WooHoo with ...". If you can get both Sims in the same bed, the WooHoo option also becomes available.
Jealousy is a curse. To keep the dry buff at bay, you may be tempted to have multiple partners, but they will tend to catch you cheating, potentially causing other negative buffs. Any partner may independently attempt a romantic interaction, upsetting others. Being in the next room is often close enough for another partner to notice impropriety. Privacy settings on rooms are generally unreliable, and will not affect partners. The exception to this is the tent: you may "WooHoo with..." in a tent with another partner standing right outside, totally oblivious.