May a wife ask for divorce because she cannot bear her husband taking another wife?

A wife may apply for separation, but the route depends on the circumstances — khulʿ, taʿliq (conditional divorce) or judicial dissolution (fasakh). A lawful polygynous marriage alone is not automatically a ground for fasakh. But if the wife truly cannot continue the marriage, to the point of fearing she cannot fulfil her husband's rights, she may apply for khulʿ (redemption divorce), like the wife of Thabit bin Qais. If the husband is unfair, neglects maintenance or causes harm, the wife may apply for fasakh or use the taʿliq. The hadith warning about women asking for divorce refers to asking 'without reason'; genuine hardship is a reason. See talaq & divorce.