What is the ruling on a wife inserting her finger into her husband's anus for mutual pleasure?

It is not permissible for either spouse to insert a finger into the anus of the other, because this involves touching impurities without necessity or need. This act may also lead to addiction and the desire for sodomy. Jurists have forbidden a man from inserting his finger into his own anus and a woman from inserting her finger into her own vagina; because the first is like sodomy and the second is like tribadism. This indicates that inserting it into the anus of another is forbidden a fortiori.