He will not marry, because as St Paul points out, those who are unmarried can really give their whole time to God and His work. A Catholic priests can thus be ready to be moved anywhere at a moment's notice, like a soldier - like Jesus, he has nowhere really to call 'home' in that sense. He even gives up the privilege of being 'his own man' - he will dedicate his entire life to Jesus, in purity and faith, always in the service of ordinary Catholics everywhere. Some priests, if they show particular gifts of leadership and defence of God's truth, may in time be chosen as bishops. But most priests live out their lives in quiet, humble service of God, as parish priests, missionaries, teachers and chaplains.
Some priests are members of religious orders as well.
It is a special and unique calling, and one which takes a young Catholic man into much prayer and thought, to discern if he has that calling from God. It's true that in some countries in the first centuries of Christianity, some parish priests were married. But they were married before they were ordained, not afterwards. Even then, it was recognised that priestly calling was special and consecrated. The Eastern churches maintain this tradition, but once again, a man cannot marry once he has been ordained.