What is the cost of a divorce attorney?
Compare that to the savings from 20 years of peeing in the yard and see what the math tells you.
Also, could you perhaps pee in the toilet and not flush every time? (Especially if you have more than one toilet so your wife can simply not use yours?
And do you actually head outside to pee, or do you just pee while you are out there? IOW, It's 9pm and dark out. You are reading your book in the cozy living room when you realize nature is calling. You go outside for that? If so, the cold air (or hot air in summer) you are letting in when you open and close the door twice is likely offsetting much of the already minuscule water savings, assuming you are heating or cooling or dehumidifying your home.
Seems penny wise and pound foolish. Or cheap instead of frugal, especially when you factor in the strife in your relationship. Pee in the shower, pee in the yard if you are out there alone--no need to mention it to the wife, though don't lie about it, and the rest of the time, pay the few pennies and use the toilet.