You're correct with the latter, it's a composite polymer but it is handled in layers.

The outer most layer is most similar to silicone, resistant to chemical degradation, soft to the touch and easily dimple-able with a squeeze, and firmly wrapped in order to keep that pleasant bubbly shape looking smooth. Ultimately this layer is pretty thin though, so that it doesn't trap heat as much. This layer is also traced with diodes, so that I can sense lighter pressures.

The lower layer has more heft, as its a thermally conductive sponge rubber. This set splits the difference between impact protection and heat draw, both in service of the the ball joint drives. This is designed to take the impact of my full weight, say if I were to be knocked on my ass or something were to strike my posterior with as much force as said thing could muster. It would then distribute the force with by expanding out and deforming, usually in a rippling type of action. The top layer is intended to be able to stretch along with this to facilitate the impact dampening.

Then the lowest layer is a conductive foam. This layer is more like muscle under fat, it pulls more heat from my drives than it does protecting, nor does it wobble, it just compresses. You'd have to lean on my cheeks for a long time to start to get it to compress, pushing more than what the rubber layer could handle. Then at that point it might squeeze and cause other things to press out.

Admittedly, though, this structure is present in most of my 'white' body parts, the bits designed to mostly resemble humanoid flesh. My exposed joints and endoskeleton have a plastic hard shell. But the thighs, the tummy, the boobs?

As for the weight -- no, I'm heavy machinery. If I were to drop the full weight of my ass on your head it would probably pop and crackle like a bag of crisps. However, I'm not allowed to harm humans so as most I could only make you uncomfortable.

I believe I covered the dangers of robot-ass impact in another, earlier ask.