Unfortunately for the RC church, the best arguments for the all-male ministerial priesthood have not been gathered into a teaching document. Much better has been the effort of the Orthodox communities. For example, one argument comes from the persistent metaphor of the Church as the Bride of Christ and Christ as the Bridegroom (this image comes from many strands of the Gospel traditions, from Paul, and from Revelation at least). If one conceives the priest as the alter-Christus, as the Catholics and Orthodox do, then the maleness of the priest is required in order for this metaphor to continue to have meaning. And this metaphor is not a minor image in the New Testament: it is central, and speaks to the very meaning of the relationship between Christ and the believing community.
I'm not sure if you're referring to a specific encyclical needing to be dedicated to the issue, but as a Catholic, I have often heard the bride/bridegroom analogy used as one of the reasons for an all-male, all-celibate clergy.