It means that even those things are are right must not be allowed to get such a hold over a Christian that they become his master and he becomes their servant. Masturbation is known to be a habit that traps and enslaves those who practise it. This habit can gain such a tenacious hold over people that it saps their energies as it concentrates their mind on sex. And the more they pander to this fleshly lust, the more they are trapped by it until they are caught up in one big vicious circle. Those who become involved in the masturbation spiral eventually find themselves fantasizing about sexual relations with imagined sexual partners and Jesus said this is sin.
Masturbation prepares the mind and body for sex.
In fact, the whole cycle begins in the mind. We must never sin against our conscience. This is a God-given guide programmed into our spirit. It is dangerous to ignore our conscience. Those who do this habitually find that their consciences become impaired and unable to recognise right from wrong.