Can an ordained Deacon who is single marry? Also, can a Deacon who becomes a widower marry and still remain an active Deacon?
Generally speaking a deacon must be married before he is ordained a deacon and may not marry after ordination:
Can. 1087 Those in sacred orders invalidly attempt marriage.
It is not absolute however:
Can. 1078 §2. Impediments whose dispensation is reserved to the Apostolic See are:
1/ the impediment arising from sacred orders or from a public perpetual vow of chastity in a religious institute of pontifical right;
The Apostolic See may dispense a permanent deacon from the marriage impediment of ordination.
From the Canon Law Society of America’s Commentary on Canon Law:
The dispensation has not been granted routinely. In a circular letter of June 6, 1997, the Congregation for Divine Worship and the Discipline of the Sacraments announced that the Holy Father had modified the “current norm which requires three cumulative and simultaneous conditions” to constitute “motivating exceptions for the granting of the dispensation from the prohibition of canon 1087.” Henceforth, “any one of the three following conditions taken singly” is sufficient “for a favorable consideration of the dispensation from this impediment, namely: (1) the great and proven usefullness of the ministry of the deacon to the diocese to which he belongs; (2) the fact that he has children of such a tender age as to be in need of motherly care; (3) the fact that he has parents or parents-in-law who are elderly and in need of care.”
If a man who is single is ordained a deacon, he is required to make a promise of celibacy (canon 1037). Thus he can not marry unless he is laicized .
A married man who is ordained a deacon but later becomes a widower may apply for a dispensation from the Apostolic See (Congregation for Clergy) to be remarried.