I have to wonder whether you intend to play nay-sayer no matter how detailed my explanations are. Of course there must be legal definitions for use in legal documents. There are laws on marriage and family and always have been in any form of modern society. In fact, what you said is really a truism in the modern world, isn’t it. If there are legal documents, there must be law and legal definitions. But that in no way avoids the problem that I explained, or justifies its creation.