Your fault? Honest opinion?
No and Yes.
You've clearly stated that they should settle the matter themselves
and should not get you involved.
It's not your fault.
You've also created this impression of a - relationship solution
agent/counsellor.
Which is why it is natural for them to come to you whenever
problems arise.
Their dependance and weight of relationship is onto you when in the
first place, it shouldn't be.
It could be possible that you've not been drawing the line clear
between counselling and friendship advice.
So on this point. Yes it's your fault for not stating clearly what
and how you can help.
From a friendship point of view. Again, you're not at fault.
_______________
If this couple's relationship is unable to keep within two person.
Their problems will always need third party to be resolved.
This r/s is seen unsustainable in my point of view - 
A relationship not
being able to stand on it's own is not a grown/matured form of
Love.
The basic foundation is soft and sinking. Either one of them are
unwilling to tie matters up, looking at facts plainly and trying to
solve it.
Weak parties + weak/miscommunication = dying r/s
It is human for us to
look at matters in a logical manner.
However when it comes to people whom are close to us who faces
problems, we can't help but to include emotions into our logical
solution. Which actually is distorted already.
Again, it's the line where you'll draw.
Cheers