It is intimately connected with the nature of the Eucharist.
It goes back much farther than most people—including the Pope, apparently—realize.
http://www.canonlaw.info/a_deacons.htm
http://canonlawblog.wordpress.com/2012/11/20/clergy-continence-and-custom/
I have always found it fascinating how exercised so many people can get against a practice that: a) is 100% voluntary for those who do practice it; b) has absolutely no effect on anyone else.
I disagree. Mandatory clerical celibacy came about many years after the Church was established and for reasons which have absolutely nothing to do with anything which can be found in the Bible. In point of fact priests were married men in the Bible and most of the early popes were married men as well as the overwhelming majority of the clergy. For further reading, I suggest:
“A Complete History of the Catholic Church to Present Day”
By Rev. John Laux.
Here is a pertinent sentence from that book:
“It is said on good authority that, in the tenth and eleventh centuries, half of the priests, in some countries more than half...lived openly as fathers of families...”
Clerical celibacy was still quite open to discussion, interpretation and practice until the Council of Trent 1545-1563. And really wasn’t tightly enforced for some centuries even after that.