huncamunca, nuns and priests are following a religious calling that overrides the command to go forth and multiply and who choose to live a life of poverty, chastity, and obedience. Clergy that have chosen a vow of abstinence, have done so as a personal sacrifice using their free will.
This wasn't always a case and nuns and priests used to marry. The 'ban' if you like really stems from the days when priesthood had become a family business passed from father to son; this passing down of the family business included the treasures of the church including relics and books. Not all sons went into the family business but they kept the treasures of the church, so they needed to put a stop to it. I don't know why they just didn't ban the inheritance, but there you go.
Personally whilst I can admire their devotion to their faith - and one of my aunts is a roman catholic nun - I do feel generally that it is a somewhat lonely life and unfulfilling in respect of having ones own family. But they do not see it that way as their company is with God and their brethren, and many couples are childless also but not through choice. So to say that a life without children is a wasted life would seem insensitive and in all honesty, untrue.