This is what my SO and me have been doing for 8 ish years now and it's great. We call it "pocket money". I can't complain when she spends it all on shoes and bags, she can't complain when I spend it all on beer and hamburgers.

When we were poor, we both only got a little pocket money each month. Now that we're richer, we both get more. I buy hotel breakfasts now sometimes (the buffet ones), on normal work days, in my home town. It's terribly decadent but hey, it's my money. Other people buy iPhones. I have nobody to answer to but me. It's lovely.

I warmly recommend pocket money.