Scientific:
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.
Laymans Terms:
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."