As a fairly conservative Catholic who believes that the male-only priesthood is correct, one of the things that bothers me is the really bad arguments that conservatives often make in order to justify the Church's practice.
The problem with most of the arguments I've heard is that they imply that Christ assumed manhood, but not womanhood.
Now, this might not sound so bad to a lot of conservatives, but having recently been studying how the early Church hammered out Christology, it strikes me as incompatible with one of the fundamental principles of Christian doctrine.
"That which is not assumed is not healed" -- Saint Gregory the Theologian
If Christ assumed manhood, but not womanhood, then following St. Gregory's principle, women cannot be saved. Obviously this is heretical. But it seems the natural conclusion of the arguments conservatives typically use.
So, the trick is to find a way to articulate the necessity of an all-male priesthood without saying that Christ assumed manhood, but not womanhood.
Perhaps rather than focusing on manhood/womanhood as if they were things that could be assumed independent of each other, we should speak of Christ assuming Human Nature as a whole, including the sexual differentiation that encompasses both manhood and womanhood.*
Of course, that might mean no longer attempting to use Christ's incarnation as male to explain the all-male priesthood. On the other hand, perhaps there is a way to do so without running afoul of St. Gregory's principle. I'm just not sure what it might be yet.
* The medieval mystics, notably Julian of Norwich, managed to conceive of Christ as both male and masculine while simultaneously feminine (though not female).