Here’s a very interesting article about why some spouses lie about money. Here it points out that lying about spending relationship money is financial betrayal.
“Financial experts, therapists and divorce lawyers point to a handful of typical reasons for financial betrayal, including addictions, affairs, abusive behaviour and differences in values”.
One great way to avoid financial betrayal is to have a Relationship Property Agreement (Contracting Out Agreement also known as a Pre-nup) that keeps your finances separate, including debts. That way if your spouse\partner is getting into debt wasting money on addictions or affairs, you are not liable for that debt. It will also keep your assets protected should your relationship end.
It will also avoid arguments over how either of you are spending money if you have differences in values\priorities because you’ve already decided from the outset that your finances are separate and you can do what you like with your money.