Sometimes there are gaps that the married person cannot fill with their partner - the partner may not feel comfortable doing some things that the other person wants. In my case I am far more highly sexed than my wife and it physically uncomfortable for her to continue after a while. Meeting with other girls allows me to "scratch an itch" and allows us to continue with our relationship without getting hung up on the feelings that would be coiled up in both of us - me for being sexually unsatisfied and her for not being able to satisfy. It is a release for both of us.