Perplexing question.

Why do all old(er) men in movies lose their ability to open and close their jaws when supposedly passionately kissing a woman? Or do they do this on purpose, so as not to gross out a younger audience?

I watched Shopgirl last night, and Steve Martin looked like he'd never kissed anything but a wall in his life.

But, I've noticed this in other movies, too. They grab the girl, wrap their arms around her, and lunge in with their mouths half open. And then for the entire 5-minute kiss in the rain, their mouth position does not change. They simply move their head from side to side, like they have a Jane Fonda workout playing in their heads.