What is masturbation, according to the Catholic Catechism?

> By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. CCC 2352 So does this mean I need to buy extra gold bond medicated powder to avoid jock itch (and mortal sin), or is that just excessive scrupulosity? The real question is, is the 'grave matter' involved in masturbation always involve ejaculation or does the offense against chastity lie in your intentions? And the source of consternation in this is that in conjugal sex, ejaculating outside of your wife is considered the 'grave matter' so I'd have to imagine the rationale for that being bad is totally different than the rationale for masturbation being sinful but I don't know why.