When I am in Sperlonga one of my favourite restaurants is Amyclae. It is placed in the lower village, at the border of the busier area: so you can have a quiet dinner there, and then reach the pretty and "alive" areas of Sperlonga at night.
(P.s.: be careful: the restaurant is placed near a hotel with the same name, but as far as I know it has nothing to do with the hotel's restaurant !!! E.g. while the hotel's entrance is on the street, the entrance of this restaurant is on another side of the hotel's building.)
The seafood served in this restaurant is from the local catch and it is always fresh and well cooked; pasta and cakes are well prepared as well.
The restaurant is good-looking though simple, and just over the beach and the sea: the restaurant's atmosphere is quiet and discreet. It is family-run, so the service is cared particularly. In summary Amyclae is a nice, clean restaurant with a good cuisine and just over the beach and the sea. Prices are not cheap, neither too expensive: I find them reasonable for the food and quality of the restaurant.