Nothing to add to Bart, I thoroughly agree. My wife and me have been sharing one bank account for the over 50 years we are living together. No asking permissions, just checking if what's left at the end of the month (or on the savings' side) allows for GAS satisfaction. And as Bart said: just a matter of mutual *trust*.
I remember when I bought my Taylor guitar twenty years ago (812 CE, even now in between 3000and 4000 $) and asked her if she thought that spending that much for a guitar is reasonable her answer was short and up to the point: "buy it or else I have to listen to your regrets for the rest of my life". Of course I've made up for it by giving her private concerts and playing on stage one tune I composed especially for her. You can listen here to the version with a friend of mine improvising to it.