I understand that masturbating for men is wrong because it spills the seed. Since women don't have see why is wrong for them.
Masturbation is not primarily wrong because sperm is wasted. It is wrong because it removes genital pleasure from its proper context–which is the physical expression of vows made at the altar. This is just as true for women as it is for men.
This is what the Catechism of the Catholic Church has to say on the subject:
2352 By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."138 “The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose.” For here sexual pleasure is sought outside of “the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved.”
To form an equitable judgment about the subjects’ moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors that lessen, if not even reduce to a minimum, moral culpability.
Fr. Vincent Serpa, O.P.