What a couple is
Not every pair of opposite forces is a couple. The definition asks for three conditions.
A couple of forces
Two forces of equal magnitude acting on a body in opposite directions, along lines of action separated by some distance from each other, are called a couple of forces.
The three conditions
1. Equal in magnitude
The two forces must be the same size. If one is 10 N the other must be 10 N.
2. Opposite in direction
One acts one way and the other acts exactly the opposite way.
3. On two different lines
The two forces must act along two parallel lines separated by some distance, not along one and the same line.
Drop the third condition
If the two forces act along the same line, they cancel each other completely. There is then no turning and no motion either. The separation between the lines is the very thing that makes a couple a couple.
The link with Newton's third law
Even when a single force is applied to a pivoted body such as a door, two forces are really at work: the force you apply as the action, and the reaction force arising at the pivot. Those two form a couple.
In other words, a pivot supplies the second force on your behalf. Apply a couple and you are supplying both forces yourself, so no pivot is needed.