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1.
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."
by Joe Herdklotz January 31, 2007