I wrote extensively about this in a post on my "Notebook" blog here
with regard to a misunderstanding we had about boundaries.
The short version of how I view it:
If either MrS or I (or Dude or I, for that matter) requests a “couple” boundary that we each agree to – once it is agreed upon then each member of the couple has the same stake in the boundary regardless of who suggested it. It was a decision that “we” made – neither member of the couple can agree to its negation without consulting with the other.
If a boundary affects another person I don't think it is unreasonable to discuss it with them - they might have a viable alternative that you have not considered - including the rationale behind the boundary so that people don't run into those "letter of vs. spirit of" situations