Nope, you don't. No one gives a shit what definitions you make up. You can call an apple a "banana," it still won't matter anymore than your definition of marriage.
You are right that society does. And our society, empowered by the Constitution, defines legal marriage as the union between two consenting people who are at least the age of consent according to their respective state's laws.
You left out “not too closely related” did you do that on purpose?