Bereavement seems to me to be a doubtful way to discern a vocation to celibacy. Maybe rule this began as a particularly stupid interpretation of  1 Timothy 3: 12 where "one wife" surely meant  "one at a time"!
I do not expect my wife to predecease me or to marry again if she did. Ho9wever, although I knew the rules on the matter before my ordination , I am glad to say that I was never asked to make any promise on the matter.
Deacon Duncan 
UK