Similar to the line of scrimmage in football. It's an imaginary line that you have to get to (through dates, flowers, begging, etc.) before a girl well let you scrump her - aka fuck her.
It can also be the crossing into an area inside or outside that you go to fool around.
Billy: I've tried everything with Beth and she won't let me in there.
Fred: It's 4th and 20. You'll never make it to her line of scrumpage dude. Just punt her away.
Dave: Check out Dan and Steph walking away from the campfire together.
Tim: He's taking her into the dark. I can still see them, still see them...and they're gone.
Dave: They crossed the line of scrumpage.