Most Muppets have neither laps nor butts: there's a hand where those things should be. Except for Big Bird, who quite clearly has a lap and a butt. I'm probably forgetting some minor Muppet who also has a complete form, but for the most part people's imaginations fill in what the camera hides.

Also, the lap is NOT the opposite of the butt. They're close but not the same thing. The lap is the front of the thighs, the upper part of the legs: it's based around the femurs. The butt is the back of the lower abdomen, centered on the pelvis: if you're sitting on someone's pelvis... well, I guess you're extra lucky today.

Unless your partner is some sort of Muppet. Then you're just a pervert.