And this is where dissenters fall apart.

What you say is true....and this is exactly what the Church says.  A null marriage.

The Church takes a couple at their word when they say that they said:  "I do". 

It's up to the couple to provide the Church with a credible reason why the marriage is null.  

That is, they must show that a "defect" preventing union existed BEFORE the marriage was declared. 

Saying later...well he committed adultery after we were married...doesn't fit the test.