Besides weight distribution and steering geometry issues, there is also the problem with a front-side passenger interfering with the operator using the controls.
The passenger's legs need to go somewhere, and the operator still has to reach the gear-shift and the rear brake pedal. A motorcycle steers at speed by leaning, but at slow speeds you need to turn the handlebars further. The space where they will go is right where the front-side passenger is located.
I ride, and have done several long trips (Northern VA to St Catherines ON) with my wife riding behind me. She nods off all the time when we're on highways and she has yet to fall off. Of course, I ride a Harley-Davidson Electraglide Ultra Limited, so she has a full backrest that cradles her to an extent, and I've got a backrest of my own, so she is held-in in front too.
I wouldn't try sleeping as a passenger on a sport bike...