Is Purneya Used in Relationships With Invalid Unions?

Purneya is a Levitical precept that has to do with sexual immorality and sexualist relationships. Purneya, throughout the New Testament, seems to always be used for this sexual immorability within invalid unions. Unions that are not real marriages to begin with. And if an invalid union, well then if he marries another woman, it would not be adultery because he's not bound to the woman in the first place.