The Sacrament of Holy Orders is what makes men into priests (just like the Sacrament of Baptism makes a person into a Christian). Like baptism and confirmation, holy orders 'marks' the soul and changes it. This really is just as mysterious as it sounds; the Church describes this mark as "an indelible character." Baptism, confirmation and holy orders all confer a distinct character on the person recieving it, a character that lasts forever--in this life and in the life to come.
The character of Holy Orders is what allows a priest to act in persona Christi, a Latin hrase that translates to "in the person of Christ." This is how the priest celebrates any Sacrament, not by his own power, but by the power of the Holy Spirit moving through him.
The Church needs priests. Without men answering this call from Christ, who will give the Sacraments to the next generation? Do you think that God might be calling you to serve Him as a priest? Learn more about how to listen to His call!
In a word, yes; the Sacrament of Holy Orders can only be recieved by men. The reason is simple: that's the way Jesus Christ wanted it. The Catholic Church only has authority as it is given by Jesus; we are only able to do what He did.
At His time, Jesus broke just about every social convention that existed. He could have chosen a few women to be among His 12 apostles. He could have chosen only women! But He didn't. In His freedom, He chose only men, He chose them by name, He chose them specifically, and He chose them for a particular role. Who are we to tell Him that He chose wrong?
The names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; Philip and Bartholomew; Thomas and Matthew the tax collector; James the son of Alphaeus, and Thaddaeus; Simon the Cananaean, and Judas Iscariot, who betrayed him.
Matthew 10:2-4