Reply To: Sephardim, Marriage, Gittin, and Cherem Rabbeinu Gershom

It’s a Gemara in Yevamos (65a maybe?) that it’s Assur to take a second wife without the first one’s permission.

Rav Schachter also always quotes the Ran that it’s Assur Min Hatorah to divorce your wife without permission, even before the Cherem.