Customary and traditional law supports polygamy 1000%. That is what needs to be elevated as the truest principle with respect to marriage within our context. I don't think the guy wants to marry her. He may have just bonked Atala a couple of times and felt he needed to keep that experience close by for a long time. He doesn't need to put his wife through all that stress at all. It's untidy, very untidy.
If the wife agrees to him marrying her in addition and that is what he wants and thinks he needs to be doing, that is between them. To each their own.
But to run her through the mud or his brother to run her through the mud in such a manner because to wants to maintain a bonk long term is untidy.