Facegasm
When you see someone's butt and your face turns into a tomato, your brain short-circuits, and you either laugh like a maniac or yell a string of dirty words.
I saw a fine piece of ass and my face turned red like a lobster. I laughed so hard I fell off the couch.
When I saw her butt, my face had a facegasm and I screamed like a banshee.
I saw a guy's butt and my face turned bright red, I started giggling like a madman.