When a pleasurable and intimate activity is always shared with one person ( e.g., a wife) to the exclusion of anyone else, that both indicates a closeness and causes further closeness [siman & siba]. If this activity is also experienced with others (or even a person alone) - in the past or the present - then it reduces both the specialness and the intensity of the experience with the wife.
I often illustrate this with the following true story. When I was about 19, a younger bochur who recently arrived in the yeshiva and who I had never met came into the bais medrash and sat next to me. He then proceeded to pour his heart out regarding very personal family issues he was struggling with. Initially, I was flattered that he somehow sensed that I was the right person to speak to, confirming my recent decision to become a therapist. Unfortunately, the next day I discovered that he poured his heart out to anyone who would listen. This certainly made his interaction with me much less meaningful.
Masturbation (and pornography) establishes sexual pleasure as not related to an intimate relationship. This reduces the potency of physical intimacy in marriage to be an exclusive and powerful tool in cementing the relationship.
Secular therapists (and some frum ones too, unfortunately) used to believe that the only possible objection to pornography (and masturbation) is a religious belief. More recently, therapists and marriage counselors have written articles in professional journals on how these activities often cause sexual dysfunction in marriage.