What episode the scene with Bart & Lisa at the cinema watching fornication metaphors?

Question

 

I just remember that Bart & Lisa are at the cinema. Homer sent them there. And at the screen are showing various images about fornication, like a rocket,  sausages, etc.  Bart ask to Lisa " what are our parents doing now?", o something similar.
(I would like to found the GIF wiht that  succession of images).
Thank You !

succession

Answer 1

It's "Grampa vs. Sexual Inadequacy"