With regard to the Priesthood, there is no difference between a vow of celibacy and a promise of chastity.
Main Entry: cel·i·ba·cy
Pronunciation: \ˈse-lə-bə-sē\
Function: noun
Date: 1646
1: the state of not being married
2 a: abstention from sexual intercourse b: abstention by vow from marriage
Main Entry: chas·ti·ty
Pronunciation: \ˈchas-tə-tē\
Function: noun
Date: 13th century
1: the quality or state of being chaste: as a: abstention from unlawful sexual intercourse b: abstention from all sexual intercourse c: purity in conduct and intention d: restraint and simplicity in design or expression 2: personal integrity
Chastity is all about sex, specifically, no sex, no way. Historically speaking, of course, one could be consummately married, i.e., non-celibate, yet later chaste by vow.
Of course, the priestly ideal combines celibacy and chastity. I also recommend it for Osprey pilots, keeps'em sharp.