"Do you think that monitor lizards pair up early in life in the wild before they are sexually mature?"

The difference is, in captivity you have a limited number of lizards available for pairing and you have a limited amount of space available for them to live. In the wild, they can court, shag, and separate at will. In captivity, they are stuck with each other in a little box for as long as you want them to be together, or until one kills the other one.