A sacrament is an outward visible sign that communicates the grace it signifies.
From the Cathecism:
1227. According to the Apostle Paul, the believer enters through Baptism into communion with Christ’s death, is buried with him, and rises with him: Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.The baptized have “put on Christ.” Through the Holy Spirit, Baptism is a bath that purifies, justifies, and sanctifies.
1234 The meaning and grace of the sacrament of Baptism are clearly seen in the rites of its celebration. By following the gestures and words of this celebration with attentive participation, the faithful are initiated into the riches this sacrament signifies and actually brings about in each newly baptized person.
1240 In the Latin Church this triple infusion is accompanied by the minister’s words: “N., I baptize you in the name of the Father, and of the Son, and of the Holy Spirit.” In the Eastern liturgies the catechumen turns toward the East and the priest says: “The servant of God, N., is baptized in the name of the Father, and of the Son, and of the Holy Spirit.” At the invocation of each person of the Most Holy Trinity, the priest immerses the candidate in the water and raises him up again.
The baptismal formula is pretty important in this regard. Baptizing in the name of the Trinity means acting by their authority, as their representative. And so women can be symbols or representatives of the Trinity. EDIT: Also note that the “priest immerses the candidate in the water and raises him up again.” Christ gave himself willingly (signified by immersion) and the Father raised him up by the power of the Holy Spirit (signified by raising the candidate up again). Women can do this, and did so on a regular basis when deaconesses were ordained in the Church (now only in case of emergency).
It is true that everybody isn’t called to the priesthood. It is also true that being a priest isn’t a right (an argument you didn’t bring up, but which is often used). The problem I have is when such arguments are used to exclude groups of people based on gender, race or other human abstractions. The same arguments could be used to exclude all kinds of groups from the priesthood. Jesus never authorized the Church to ordain blond and blue eyed males. A racist could argue that “nobody has a right to become a priest, and so it would not be discrimination not to give the priesthood to such males. Jesus had brown hair and brown eyes, so how can a blue eyed blond male represent him? He would make a poor icon of a Jewish Messiah…”
Since any vocation to the priesthood is not merely a personal thing (where one feels called to become a priest by God), but also a communal thing, where the Church must also vouch for the vocation, one could easily exclude all blond and blue eyed males who felt they were personally called to be priests by Christ. They might subjectively feel that they have a vocation, but without support from a (discriminating) Church, they never objectively have, or so we are told.
To me, the women-priest thing is not primarily a women rights issue, although I do feel women are being discriminated against for no good reason. I do, however, feel it is a profound loss for the Church that so many wonderful and qualified women who could do a world of good for the Church, and bring a much needed feminine perspective into the higher ranks of the Church, are not ordained. This comes from experience of women ministers in other religions who are deeply spiritual, conduct the sacraments of their religions with deep reverence, and who clearly have an awesome gift to teach others with clarity and compassion. But of course, it is not my business to tell the Church who to ordain. I have no such authority. I am merely expressing my opinion based on my admittedly limited knowledge and experience.