Is all touching of the genitals prohibited to spouses?

No. However, touching the genitals of yourself, or of your spouse, in the same or similar manner as would be done in masturbation (i.e. manipulative sexual acts) is immoral. Any type of masturbatory touching is immoral (regardless of whether or when sexual climax occurs) because it is a sexual act that is non-unitive and non-procreative.

Any claim about the morality of an act which "dissociates the moral act from the bodily dimensions of its exercise is contrary to the teaching of Scripture and Tradition." (Pope John Paul II, Veritatis Splendor, n. 49.) In other words, the concrete act (the type of behavior) cannot be separated from its inherent moral meaning. An intrinsically evil act cannot become moral merely because the will directs the act toward a particular purpose. Certain kinds of acts are inherently immoral by the very nature of the act. And so, masturbatory touching of yourself or of your spouse does not become moral by being done within marriage, nor by being done to prepare for the marital act. The bodily act itself cannot be dissociated from its inherent moral meaning. So when an act, such as masturbation, is intrinsically evil, it can never become moral, not with any intention, not in any circumstances, not within marriage, not in association with the marital sexual act.