A leap frog is a person in a boyfriend/girlfriend relationship who uses their current boyfriend/girlfriend to "leap frog" to a potentially better, more attractive, or more popular boyfriend/girlfriend.
Leap frogging can be considered a form of betrayal and infidelity.
Hunt: Janine dumped me yesterday.
Sean: Ouch, harsh.
Hunt: I think she was just using me to leap frog. I saw her holding hands with another guy just today.