Summary of Clerical Celibacy Rules:
All churches expect chastity from their clergy – i.e., either be sexually active only within a marital relationship, or be celibate.
Most Protestant churches: One rank, Pastor, may marry, before or after ordination.
Methodists: Two ranks, Deacon and Elder. “Bishops” are Elders named as General Superintendents of the church. All may marry, before or after ordination.
Anglicans: Three ranks: Deacon, Priest, Bishop. All may marry, before or after ordination, unless part of a religious order (rare) which is vowed to celibacy.
Orthodox: See rocking chair and ybeayf’s posts. Three ranks, as with Anglicans. Deacons and Priests may marry before ordination unless part of a religious order (common). Bishops are chosen from senior members of religious orders and are celibate. (I believe the “Oriental Orthodox” – the five non-Chalcedonian churches which ybeayf mentioned as “otherwise” – have exactly the same rule.)
Catholicism: Broken into Roman Rite (overwhelmingly the largest part) and Eastern Rites.
–Roman Rite: Deacons may marry before ordination. Candidates for the priesthood may not marry, by Church Law (not doctrine) unless extraordinarily dispensed by the Vatican. Married clergy converting to Catholicism may be ordained and remain married. Priests wishing to marry are laicized, i.e., remain priests but are deemed laymen, and may not function as priests except in emergency situations.
–Eastern Rites: Same standards as for Orthodox.
Note that Orthodox and Eastern Catholic deacons and priests and Catholic deacons may not marry. Married men may become clergy, but single or widowed clergy may not become married. Protestants and Anglicans do not have this rule.