What is the argument against the imposition of clerical celibacy as practiced in the Roman Catholic Church?

The Roman Catholic Church requires its priests to practice celibacy. This has not always been the case, though. In fact, it was not instituted until the 11th century. Protestant churches have no requirement of celibacy. So, what is the reason for this? Specifically why do Protestant churches, which began well after the practice had been instituted in the Catholic Church, do not require this of their pastors?