Whether a corporation is a “legal fiction” depends on the meaning assigned to the term. As the term is used by anti-capitalists, “legal-fiction” is construed to mean that the owners and managers of a business entity become rightless serfs once they have assumed corporate status.
Under capitalism, the name of a corporation is made up (fictional), but the charter underlying the corporation is protected by right.
A marriage may have a legal framework behind it: this does not make it fictional. The same principle applies to the concept of society, and the same goes for a corporation where real individuals organize together under a fictional (i.e., made up ) name.