It seems to me it would have been less appropriate for God to ask Sarah to sacrifice a daughter than to ask Abraham to sacrifice a son. I don't have an argument for this—that's just how it seems to me. But if this is right, then it is not an accident that in the `Aqedah (the binding of Isaac) the two persons involved are male. But the `Aqedah is a foreshadowing of Christ's sacrifice on the cross. While with respect to the Incarnation as such, Christ's maleness might be reasonably argued to be incidental, if my intuition about the `Aqedah is right, then it is plausible that with respect to the sacrifice of Christ, the maleness is not incidental. And if so, then since what is central to the priesthood is the offering, in persona Christi and in an unbloody way, of the one sacrifice of the Cross[note 1], it seems quite appropriate that the priest be male, since he represents one whose maleness is not accidental in this context, and participates in Christ's sacrificial activity to which activity Christ's maleness is not accidental.
Is this sexist? Here is a way of thinking about this. Suppose that part of the reason God asked Abraham to sacrifice a son rather than asking Sarah to sacrifice a daughter had to do with Abraham and Isaac's maleness (leave aside the accidental fact that Sarah perhaps didn't have a daughter, since God could easily have fixed that). Would it follow that God discriminated against Sarah in asking Abraham to make the sacrifice? Surely not: one can at least equally well say that it was Abraham who was discriminated against by being asked to make the sacrifice.[note 2] The restriction of conscription to males does not discriminate against women, but against men, since it is upon men that it imposes a duty that it does not impose on women. Similarly, if God restricted who he requires to become priests to men, it is not obvious that this would be a form of discrimination against women.