Oy, there’s a tricky question. I’ll answer it as best I can, realizing that my short theological diatriabe here will probably fall far short of a satisfying answer for anyone.
It requires, from the start, an understanding that God did in fact create two sexes, and that like it or not, men and women are different. (It’s way back in Genesis: “God created them male and female.”) There’s certain things men can do that women can’t, and vice-versa, and even if we consider that marvels of medical science could eventually create child-bearing men, there’s a specific “role” capacity that makes a man a man and a woman a woman that is ingrained into the soul.
(And this will probably be the biggest sticking point here, due to the endless gender discussions I’ve seen on the boards and whether it’s an immutable trait or something chosen by the individual. Point being that from the church’s perspective, maleness and femaleness are not just physical characteristics, but spiritual ones.)
It’s been speculated that the gender division is meant to be a biological metaphor for God’s relationship to humanity. Seriously, some theologians refer to all of creation being “female” (because it produces) with God as “male” (as a creator). This, by the way, is why God is referred to in the Christian as a “He”: not because of biological function, but because of nature. (And that’s a whole other can of worms. This is not the argument you’re looking for. Move along.)
Then comes Christ, who again, is male. While the question is frequently raised that Christ could have been incarnated female, point is, he wasn’t. (Side note–I often see it speculated that Christ wasn’t female for “practical” reasons–i.e., nobody would listen to a woman back then. My best response is, gee whiz, the vast majority of them didn’t listen to him anyway.) Some of the Pauline letters again use the gender imagery: Christ is the bridegroom and the church (humanity) is the bride. Christ provides everything for the church as a good husband would, and the church is meant to return its full love as a good wife would.
The important point is that the priesthood–and this is tricky, but important–is meant to be a “duplication” of Christ. (I don’t have a better term here.) That is, the apostles and those to whom they passed their power have the authoritative powers of Christ. Yes, I’m aware that most priests don’t walk on water, but they nonetheless stand in Christ’s place. Hence, priests can forgive sins, transubstantiate bread into Christ’s flesh, etc. (There was a thread on sacraments a few weeks ago where I and some others did our best to explain the priestly powers.)
So why a male priesthood? The short of it, as best as I can explain without digging through every JPII book I have, is that priests are not just imitating the function of Christ, but his nature. “Maleness” was one of Christ’s essential features: it goes not just to functional fulfillment, but to the core of his being. (Best I can do to illustrate this on the spot–a single mother is probably going to have to play both father and mother to her children. Despite taking on a father’s function, she is not in and of herself a father by nature.) Women could, of course, fulfill the ministerial functions of the priesthood (preaching, teaching, etc.), but there would be a specific essence lacking.
I realize there may be a lot of flak in the responses to this posting. For a more detailed answer, I’d suggest reading anything on the Catholic church’s position on gender. John Paul II’s “Theology of the Body” might be a good start, but papal writings tend to be thick, so look for some kind of supplement to go with it.