The guy takes the passenger seat behind the woman as she leans forward onto the handlebars to expose her coochie. If the bike has a sissy bar, he can lean backward onto it while tilting his pelvis up. Otherwise, he may need to grip the sides of the seat to get leverage for thrusting. She can also ride him by bracing her feet on the pegs and moving either back and forth or up and down. This position is good for a controlled fuck - just make sure she doesn't give it too much throttle when she gets turned on.