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.
Which acts of foreplay are moral?
Any act is moral if the intention and the moral object are both good, and if the good consequences of the act outweigh any bad consequences. If the intention and consequences are good, then the morality of an act of foreplay will depend on the moral object. Any sexual act which is non-marital, or non-unitive, or non-procreative is intrinsically evil and always gravely immoral, even if used for the purpose of foreplay, due to the deprivation in the moral object of the marital or unitive or procreative meanings. However, acts such as a husband kissing and caressing his wife’s breasts, or a wife kissing and embracing her husband passionately, are not sexual acts per se, and so these acts are generally moral and do not need to be unitive and procreative.
In order to be moral, each and every sexual act must be unitive and procreative. Non-unitive or non-procreative sexual acts (i.e. unnatural sexual acts) are intrinsically evil, and do not become moral by being used as a type of foreplay, nor by occurring before, during, or after an act of natural marital relations.