I never quite got that one!
I always assumed that every couple would have a joint account for the household (including any children), and separate accounts for their own spending money. Providing that both partners paid their fair share into the joint account, then what they did with their own money would be their own business.
(Okay, I can see that blowing large amounts of cash on drugs and/or call girls/guys might not go down well, but otherwise personal purchases shouldn't be a problem!

)