Supergirl is supposed to be soft and feminine, yet at the same indestructible. How, you may be asking, is this seeming contradiction possible? How can you bounce bullets off of the same skin that would feel smooth and pliable to the touch?
Well, I think I know the answer. And that answer can be found in … CORNSTARCH.
You see, when you mix cornstarch with a little bit of water, it becomes this mushy, pliable mixture. You can stir it and press it with no problem, it’s soft and pliable and yielding. However, if you strike it hard it suddenly becomes extremely rigid. It gets an unyielding, very very solid surface to it for a fraction of a second, then “relaxes” again. If you hit it hard enough, you can see cracks in its surface, just like cracks you’d get if you punched drywall; and then these cracks fill in with the liquid cornstarch solution and everything goes back to normal.
I’m betting it’s the same way with Supergirl. Punch her or shoot her, and she’s the Girl of Steel; hold her hand or kiss her, and she feels like a soft-skinned human woman.
Note: This would also explain why Lois Lane can still enjoy it when she kisses Superman.