Imam al-Shafi'i (may Allah have mercy on him) said: "Because Allah, the Mighty and Majestic, made it unlawful only on account of the sanctity of the lawful (relationship), and the unlawful (relationship) is the opposite of the lawful."
He said: "And our view has been narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both)."
Yahya bin Ya'mar narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning a man who committed zina with his wife's mother or her daughter: he has transgressed two sanctities, but his wife does not become unlawful to him. Yahya bin Ya'mar said: The unlawful never makes the lawful unlawful. This reached al-Sha'bi (may Allah have mercy on him), and he said: Rather, if I took a jug of wine and poured it into a cistern of water, that water would become unlawful. Al-Sha'bi's view was that she does become unlawful to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13960