Hey Andrew,
This is just a mix of my own thoughts and things I can loosely recall; I might be completely off on all of it, but:
I think this is a much smaller deal than feminists and people for equality like to make it. Anyone can be a wonderful person, a great Christian, and an exemplary follower of Christ without ever being ordained. A good example of this is the vast number of female saints. Saint Mary, the greatest created human being ever, was a woman. (If you identify this as a false dilemma, you’re missing the point.)
However, I believe men-only-priests isn’t something God revealed to us, it’s something
we decided. I.e. It could change if we decided to change it.
I think the Church’s reasoning went something along the lines of: women tend to be better caretakers of children, and thus are better suited for the vocation of marriage, which is just as holy and just as Christian as ordination. If Mary was the perfect woman, note that she was nothing if not a mother. Not to put any fellow men down, but I think the woman is the more important figure in the sacrament of marriage than the man, even if the man is the family leader. Men tend to be more philosophical and more zealous in their work, and thus are better suited to the vocation of ordination, aka doctors/ministers to God’s (adult) people. If Jesus was the perfect man, note that He was nothing if not a minister. (Please don’t be a SA and take this literally–you know what I mean.

)
The Church also (I think) reasoned that being truly dedicated to two life-vocations at once would be difficult to impossible. Thus, even if we decided to start women priesthood too, I think there would would be some rule, like that of men priests, stating they could not also be married. Now this seems contrary to the main purpose of womanhood, thus, only men are ordained.
But, as your debater said, some women do make good leaders, or fulfill the fatherly role well. So maybe someday we, the Church, really will decide to allow women’s ordination.
My main point is: selectivity in ordination is pretty much up to us. And: discrimination in this and in many other things isn’t necessarily hateful or unfair; it can be a good thing.
-Greg