Romance is one of the five main interactions, and can only be used on a sim who's "in the mood". Along with the Impress interaction, it's the only one not to be used as part of the main goals.

When performing the interaction sims will make one of three gestures which indicate that you have to do different things. If they put their hands together and lean forwards, you kiss them, if they signal to come towards them with their hand at their side you show off bod, and if they put their hand to their ear you serenade them.

Some sims can't be romanced, these are Lord Mole, Optimum Alfred, Bigfoot, Jeb, Pepper Pete, Emperor Xizzle, and Horus. Sims of the same gender can't be romanced.