Untenable - use of the word
I found a sentence:
"The situation in their marriage became untenable"
I checked the meaning of word "untenable" and the dictionary says that it's only:
"incapable of being defended or justified"
Is this word used correctly in this context? Wouldn't the word "unbearable" be more appropriate?

Question

Answer

untenable means "cannot be maintained".

Answer

In this sentence untenable means impossible
  "The situation in their marriage became impossible"

Answer

'Untenable' is used suitably for the sentence you have said as an example. 
The sentence : "The situation in their marriage became untenable" says that their relationship for certain will fall apart; there is no way so that they can still live together as a wife and husband. 
in return adjective ' unbearable' we can use  e.g,: an unbearable pain (= the pain makes you upset and annoyed)
Correct me if my way of apprehension is wrong;)