well when the car is at an idle... and the car is not moving, of course the engine is going to get a little hotter... as there is not any air flow to cool the engine. When you turn the A/C on, the fans kick on, cooling the engine down.

Just check your coolant, and make sure that the fan turns on automatically when the temperature rises... if not, the fan temperature sensor or relay maybe bad. Only one fan turns on when the temperature rises, both fans come on when the A/C is on.