When a man inserts his penis into a woman's vagina, and begin coitus. Within minutes of insertion, both are tired and stop intercourse, realizing that rather then the intercourse taking hold it has slipped away.
It is the opposite of sex that sticks. It doesn't take hold, or stay with you. Ergo, rather then the sex sticking, it doesn't stick.
Bob and Sally are having sex
Bob: "Oh Dear God! This sex isn't sticking"
Sally: "What do you mean its not sticking?"
Bob: "Its like sex that sticks, but it doesn't"
Sally: "Oh My, so thats Sex That Doesn't Stick."