You want to require this form because without it people will not have medical proxy power for their spouse. Even when married. That undermines marriage. Marriage means one flesh.
Oftentimes the hospital is an interested party in this. IIRC, in the Karen Ann Quinlan case, her mother (she was unmarried) and the hospital both went to court for an order to allow her respirator to be removed, as neither wanted to be guilty of murder.
As far as the "one flesh" - that may be Biblically correct but in the secular world you are two separate entities.
If they are one flesh in the way you suggest (as opposed to God's meaning), when one dies, so does the other. So, where are Michael's ashes?
You can call them "one flesh," but what you have described is owner and property. Even property owners have certain responsibilities and limitations. You can own a dog, but you're not allowed to starve and dehydrate it to death.
When a man and woman are one flesh in the way God intended, one does not kill the other. God intended the bond between husband and wife to go much deeper than the superficial set of circumstances you have laid out.