What is the scriptural basis for clerical celibacy?

In the Catholic Church, priests are required to stay celibate. *(As far as I know, protestant priests that convert to Catholicism are allowed an exception -- they can become Catholic priests even if they already have family.)* I cannot understand how this fits with the Bible, as 1. Peter was married (Matthew 8:14 ), and he still was (according to Catholics) the first Pope. 2. According to Paul, *overseers* and *elders* must be "the husband of one wife" (1 Timothy 3:2 , Titus 1:5-6 ) 3. Paul even seems to consider the ability to raise children as a criterion of *overseers* (1 Timothy 3:4-5 ) What, then, is the biblical (or other) basis for clerical celibacy? How are the passages I mention consistent with this?