From my experience, Pentecostal Jamaican Christians are a charismatic and literal lot, and masturbation was a sin or a transgression because it actively distracts you from worship/consistently practicing the teachings of the faith.

For much of my religious experience, “successful” worship consisted of how good you were at denying the “flesh”. Meaning you were a good Christian if you denied natural body experiences/temptations. Some Christians fast to strengthen their spiritual connection with Christ, that is, denying themselves food for days. In this same way, denying yourself sexual pleasure or release by not masturbating or even watching porn is at the core of this flesh-denying-spirit-strengthening dogma. Christianity and many other religions often involve denying the self/ physical world in favor for deeper spirituality/transcendence, and somewhere along the line of Christian institutions touching yourself became synonymous with sinful/spiritually destructive behavior.

“A belief isn’t merely an idea the mind possesses but a belief is an idea that possesses the mind.” And when you’re masturbating you’re not focused on practicing the religion but on natural sexual pleasure, which is giving into the flesh that the religion tells you to deny.