What is the ruling concerning a person who engages in intercourse based on a corrupt marriage contract while believing it to be unlawful?

Primary text

If an individual engages in intercourse based on a corrupt marriage contract while believing it to be unlawful, they incur sin (*ithm*) and are subject to disciplinary punishment (*adab*). Paternity (*nasab*) of any child born from this union is established in this scenario, just as it is established when they believed it lawful.