Priests are married to the Church.
If sex is a biological necessity, then a lot of guys on this board should be dead by now.
The problem is people confuse the role of a priest with that of any other Christian minister, and that simply isnt the case. It's the same reason in many respects that there are no female priests.
The priest, most especially in the Eucharist (but also in other sacraments such as confession) act in what is known as in Persona Cristae (in the person of Christ). He in effect becomes for us a physical conduit for Christ on earth,
The priest take oaths of obedience and chastity (sometimes poverty) in imitation of Christ. He sets himself to live by Christs example, and devote himself completely to God. In essence he rejects the things of this earth in pursuit of Holiness.
A husband by necessity must be devoted to his wife and children, and that is how it should be.He fulfills his love for God in service to his family. A priest devotes Himself to serve only God, and in so doing devotes himself to serve all. His chastity is a gift to God, and that act of sacrifice is done as a gift to all humanity (Catholics unlike other Christian denominations believe in works coupled with faith, not just faith alone). There is value in sacrifice.
The reason that there are no female priests is because the Church is bound by the example of Christ. Christ chose no female apostles, and by tradition the Church doesnt either. More importantly, God chose to manifest Himself on earth as a man, not as a woman. The priest in his function as a "step in" for Christ, so to speak,because of that uses men in the role of priests. In short, Christ wasn't female, and the Church has no authority to say to God "well he should have been"