The phrasing does point to an assumption. There is a focus on the physical difference between men and women. Like that is all there is. If you want to understand what it means to be a man or what it means to be a woman all you have to do is study male and female genitals. What more could there be?
On the other hand the formation of the question is valid. A man that has been castrated can't be ordained a priest. So in some sense a penis is required. That rule goes all the way back to the levitical priesthood established by Moses.
To answer the question you need to understand that priests need to impregnate us. That God 's presence in the Eucharist and God's presence in His word is supposed to do to us what a man's seed does to a fertile woman. That is that God's gift of Himself combines with her gift of herself and becomes a new creation. That is what we do. We don't just let God do His thing with us. We give ourselves so God's thing is also our thing. Yet it is not completely ours. It has a life of its own.
Then we take that gift into out most intimate space and nurture it without anybody noticing. We allow it to grow deep in our hearts through prayer and contemplation. Eventually it grows. We start to recognize it. It is a piece or art. It is a book. It is a ministry you want to get involved in. It is a relationship you need to take in a new direction. It is whatever God intends to give birth to through you.
If you understand this idea that we are to be sacramentally impregnated by our priests then it makes more sense why a priest needs a penis. Yes, God could set things up so that a female could become a priest and spiritually impregnate us. There would not be any physical problem with that. Yet God chooses to line up the physical world with the spiritual world. It gives us a better chance of getting the spiritual stuff right.
Peter Kreeft puts it this was:
In the very act of self-surrender to God there is joy. Not just later, as a consequence, but right then. It is exactly like a woman's voluntary sexual surrender to a man. The mystics often say all souls are female to God; that's one reason why God is always symbolized as male. Of course it's only a symbol, but it's a true symbol, a symbol of something true.