The first legislation requiring celibacy appeared as late as the fourth century (300s), but this is the first legislation that has _survived_. Until this time (ca. 314) the Church was persecuted, councils had difficulty meeting, and their legislation has been lost. Moreover the legislation of the early 300s describes clerical sexual abstinence, or continence, as a long-established practice, not something first being mandated at this point.
Even married priests were expected, according to this legislation, to abstain from marital relations with their wives once they were ordained. The woman thus exercised a veto power over her husband's ordination. The early legislation admonishes priests who promised continence but were not practicing it, to keep their pledge.
In other words, abstinence from sexual relations for both married and unmarried priests was well established practice long before the first surviving legislation in the early 300s. _It may well be of apostolic origins_ (see Charles Cochini S.J., Apostolic Origins of Priestly Celibacy [San Francisco: Ignatius Press, 1990] as well as the more recent work by Stefan Heid [1997]).
The same rules about mandatory sexual abstinence for priests were shared by both East and West until the East modified it slightly in the 690s. Even that modification was restricted in scope: priests were still required, n the East to abstain from marital relations on the days on which they “handled sacred things.”
The claim that one of the fathers of the Council of Nicea, a monk-bishop named Paphnutius, _favored_ married and sexually active priests is based on a pious legend, according to the detective work of Cochini and others. For centuries this has been taken as giving great antiquity to the Eastern position. In fact, the modified policy at the Synod of Trullo (not an ecumenical council) in the 690s was an innovation, which is why the bishop of Rome rejected it.
The assumption that bishops and priests should not be married in order to devote themselves sacrificially to the service of Christ rests on Mt. 19, 1 Cor 7 and the passage in the epistle to Timothy in which St. Paul says a bishop should be the husband of one wife (that is, should not remarry after being widowed, which was a counter-cultural requirement in Graeco-Roman culture but demonstrated self-control and a desire to devote oneself to God, as was also true of women who chose not to remarried after becoming widows and were thereafter supported by the Church on the official "rolls" because their prayers and service to the poor etc. was made possible by their choice not to remarry).
It was a practice, but it was obviously not well-established. For the next 800 years, Popes were emphasizing continence and celibacy for priests precisely because these were not being observed. It was not until Gregory (I forget which number) invalidated the attempted marriages of clerics that mandatory celibacy was universally observed in the Latin Rite.