It is Jesus who describes the very serious consequences for those who are unlawfully married.
Jesus describes in the Sermon on the Mount that the standard expected of his followers extends to the heart.
Mat 5:27-28 ESV “You have heard that it was said, ‘You shall not commit adultery.’ (28) But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
He immediately follows this command with a solemn warning that such lust, which often involves eyes and hands, will lead to eternal destruction.
Mat 5:29-30 ESV If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell. (30) And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell.
He then immediately follows this solemn warning with his teaching on divorce and remarriage, which He calls adultery.
Mat 5:31-32 ESV “It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ (32) But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
The connection between all three passages is horrifying plain. Remarriage which results in adultery and is born of lust for a person we cannot have will send one’s ‘whole body into hell’.