It is the norm for all levels of clergy to remain celibate (unmmarried), however, this has been relax for the permanent diaconate. Some deacons are allowed to remarry with a proper dispensation, for instance if he becomes a widower with young children to care for.
However, it is otherwise simply an expected part of the vocation that one will devote his entire life to the service of the Church should his wife predecease him.