(A) Are the couples, man and woman, living together openly (so people know) doing any sin? Particularly, if they are serious with and are loyal to each other and own and care for their children?
(B) What is marriage? Isn’t it declaring in front of at least two adults that the couple will from now on live together as husband and wife? And people know them as couples. Also own and care about their children.
(C) What is sin. When a man and a woman do the thing in hiding and act as if nothing happened in front of others. The children are disowned.
The only thing missing in (A) is a specific declaration, and the witnesses, as compared to (B). But in (A) the intention is declared by action and not words.
(C) lacks both the declaration and the witnesses. And also lacks responsibility of the consequences.
Your comments please
(The above is only my observation and curiosity, and do not form my opinion. I will give my own opinion later)