This is the primary reason to avoid masturbatory activity. It is rarely, if ever, done without pornography, (mostly men) or fantasizing, (mostly women). Sexual gratification stimulates the pleasure center in the brain, the same way that certain drugs can. The desire is insatiable and anyone with a masturbatory habit, can soon find themselves in a loop or a cycle, which consists of acting out, feeling guilt and shame, repenting, and then repeating the cycle. This is not simply pleasure, but an addiction.
Granted, some are more predisposed than others to having an addictive personality, which can make them more susceptible to developing an addiction of some type, but the behavior itself, whether for recreational pleasure or by compulsion is no less sinful.
There are a number of triggers which can produce a desire to masturbate, anxiety, stress, boredom etc... , but those with a serious masturbatory habit, need little provocation, before indulging.
Each individual, whether male or female, develops a particular affection which produces sufficient arousal. The problem is, for those who have an addiction, their prefered methods for arousal may change. One problem which is usually present in those experiencing masturbatory habits, is a poor self image.
Sexual Fantasy and pornography are supported by lust, vile affections, unnatural affections, inordinate affections, and a number of other types of sexual immorality. Any Christian considering these types of affection, acceptable, need only open the scriptures. There is freedom for those who desire it, but not for those who defend their sin.
Colossians 3:5
... Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry.