What is the basis for clerical celibacy?
Why it is said to be a bad thing since Christians say that God had a Son? Why do humans deny that to themselves and accept it for God?
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It's very simple actually. The Pope can't have children because in Western Catholicism, Priests are expected to be unmarried and celibate.
Mary and Joseph had a child despite celibacy, but they were at least married (thus giving Christ a legitimate father). I can't see how someone who cannot have an illegitimate child (celibate) or a miraculous child (not married) would be able to accomplish this, but I suppose that anything is possible through Christ.
The truth is that there were Popes who had children.
Pope Paul III (1534–1549) held off ordination in order to continue his promiscuous lifestyle, fathering four illegitimate children (three sons and one daughter) by his mistress Silvia Ruffini. He broke his relations with her ca. 1513. There is no evidence of sexual activity during his papacy. He made his illegitimate son Pier Luigi Farnese the first Duke of Parma. Pope Pius IV (1559–1565) had three illegitimate children before his election to the papacy.
Now, to be fair, many of these were in 1400's, a time that I suspect even most Catholics would now call the low point of the papacy, especially in theological terms. There were three Popes at one time, all sorts of politicking, and in general, the Papacy was just another earthly Kingdom. (Again, I want to keep stressing even modern Catholics would probably agree with me on this.)
Additionally, the first Pope, Peter, was at least known to be married. (Jesus healed his mother in law, after all).
Celibacy is not a "modern" innovation, but it is clearly post-Biblical.