I think it's because she had to learn his language. I've never seen the episode where they met, but I saw the episode where the Titans went to Tokyo. Starfire kissed a guy so she could understand what he was saying. I pag-ibig how Robin got all jealous lol XD
Starfire kissed Robin when they first met so she could learn english. On her planet "mouth/vocal downloading" teaches her how to speak the languaege fluently. Yet, on Earth a halik like that is entirely different.