While this appears to be a simple question, there are a few misconceptions that need to be addressed. Many Protestants-and even Catholics for that matter-do not know that there are many rites within the Catholic Church that allow married men to become priests. Though the Latin (Western) Rite practices the discipline of priestly celibacy, most of the Eastern Rites allow married men to be ordained.
Even within the Latin Rite, the Church has made exceptions for a number of converted married ministers to become ordained. This is known as the "pastoral provision," and it demonstrates that clerical celibacy is a discipline, not a doctrine. The doctrines of the Church are teachings that can never be reversed. On the other hand, disciplines refer to those practices (such as eating meat on Fridays) that may change over time as the Church sees fit.
Further, the Early Church consisted mostly of adult converts, and, of course, St. Peter was a convert. Naturally, the majority of the hierarchs of the Early Church were married men. The advice to Titus to ordain priests who are “husbands of one wife” reflects that.
It is noteworthy that while we have examples of married men becoming priests and bishops, and even popes, we have no examples of ordained clergy marrying and remaining clergy. We cannot even be sure if the marriages that we know about were not so-called josephine marriages, when the household was shared but not the marital bed.
In the case of Peter, we actually don’t even know if his wife was alive at the time of his ministry. All we know is that he had a mother-in-law, hence was once married.
St. Paul was celibate and praised celibacy in his letters, as a vocation especially well suited to ministry.