I respond to this like most things from my own experience and my own lens.
As a young unendowed man that had not yet served a mission, I struggled with masturbation. I visited a temple and walked the grounds. I committed to stop and wrote into my journal that unless I stopped I would never find fulfillment with a real committed relationship - my deepest desire. I worried that it would make me sexually insatiable and driven to ever more extreme attempts at satisfaction.
After my endowment, I felt that my garments were like a holy suit of armor (probably influenced by that seminary video "Whole Armor of God". Or that God had given me impenetrable castle walls. When I eventually succumbed again to the temptation of masturbation, I felt that I must have let the enemy in through the back door. I had betrayed the gift that God had given me. I did not have a house of light and order, I had a house of shame and betrayal ... and it was my own fault ... my own weakness.
I felt that it disqualified me from being marriage material to young women in my community - an unacceptable result. To marry outside the faith and the temple would be so disappointing to my family. It would have to remain hidden and secret.
But when sexual stimulus comes in the form of masturbation, completely devoid of the sharing and vulnerability and complementarity of marriage, then the brain can become wired so that it is primarily masturbation that produces the reward, and an individual can become increasingly unable to sexually respond to a spouse.
I felt that I would be deviant, perverted, evil. My only choice (because I was too week to stop for good) was to live a lie.
Anyone fettered by this weakness should abandon the habit before he goes on a mission or receives the holy priesthood or goes in the temple for his blessings.”
Yup, I tried that. I felt that getting endowed was my "do or die" moment. I failed.
And this harem [of imaginary brides], once admitted, works against his ever getting out and really uniting with a real woman.
I interpret this quote not as saying that someone that masturbates would never have sex or even never to get married - that would be demonstrably false. I interpret it as saying that someone that masturbates would be impeded from becoming "one" with a spouse physically, mentally, emotionally. Ironically, the secrecy that I lived by helped this to become a self fulfilling prophecy. I remember having a fight with my wife over something unimportant. I remember thinking something like, "Wow, she is really mad or hurt over this little thing. Imagine how she would respond if she ever knew the real me. She would reject me mentally and emotionally if not physically. She must never know." Because my wife could not know all of me I was denying her the opportunity to accept all of me. I knew this and still it seemed better than the alternative. Better that she love my portrayal of myself than possibly reject the real thing.
It is not anything so wicked nor is it a transgression so great that the Lord would reject you because of it, but it can quickly lead to that kind of transgression. It is not pleasing to the Lord, nor is it pleasing to you. It does not make you feel worthy or clean.
The Lord had not fully given up on my or rejected me, but it was only a matter of time. My habit could "quickly lead to that kind of [unforgivable]transgression" and because I could not stop myself, I was on a one way train.
"It is not so much the pain and suffering of life which crushes the individual as it is its meaninglessness and hopelessness." C. A. Elwood
“It is not the function of religion to answer all the questions about God’s moral government of the universe, but to give one courage, through faith, to go on in the face of questions he never finds the answer to in his present status.” TPC: Harold B. Lee 223
"I struggle now with establishing my faith that God may always be there, but may not always need to intervene" Heber13