I can't count how many times I've said something to that effect to my wife, and her to me.
Why would you have an opinion in a matter between my wife and I?
"Saying something to the that effect" to each other is not necessarily legal and binding. Get it in writing. Just in case.
Why would you have an opinion in a matter between my wife and I?
I don't have an opinion on a matter between you and your wife and I won't as long as her desires are clearly written and/or the circumstances surrounding your desires are not suspect.