Duck Butter

When two (ugly) people (male&female) have sex, and a gelatin/butter substance is made between the two of them. Usually disgusting and smelly, and leaves a snail trail like substance on sheets. If they do not separate, the duck butter will harden and cause trouble for the both of them.

"Ewww, they are going to make Duck Butter together"
"They are so ugly they make duk butter, when they have sex." - A random uncle.
"Lets go make some duck butter!"