Addendum to the eternal search
It generally does not. The people in the relationship do still compare and contrast, this time with what they settled for and what they see outside. If the relationship is strong, then this momentary comparison does not lead to anything and they remain faithful to the relationship. However, there are times when the temptation is overbearing and people stray. It is after all human to compare and want something better than what we have.
The grass is always greener on the other side.

