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What? I didn't understand anything! Polygamy??!! If you are talking about genealogy: Nurahyrion (now the old man) married a woman (that chick with healing powers we saw at Kyoto flashback, forgot the name;, his son Nura Rihan married a youkai but she couldn't get pregnant and left him, then some yeras later he married a woman who gave birth to Rikuo, then Rihan died and Rikuo became the third generation of the Nura clan!
２ 孫 mago grandson 3 曾孫 himago great-grandson 4 玄孫 yashago greatx2grandson 5 来孫 raison greatx3grandson 6 昆孫 konson greatx4grandson 7 仍孫 jouson greatx5grandson 8 雲孫 unson greatx6grandson
After that the word used is x代目の孫, (number)-grandson, i.e unson is 8代目の孫, 8th grandson.
I'm not Japanese so I don't know what they think about that, the law that prohibit polygamy itself written in Meiji era because of western influence, before that polygamy is common. Modern Japanese may think that monogamy is the norm now and can't think of polygamy (it's against their law anyway).