SO wants my blessings to cheat

Came up during discussion and partner basically says he needs my blessings to sleep with other people (pictures included). I'm so heartbroken as I assumed our relationship would be monogamous. I can't deal with it. Don't know if I should be sad or angry. Or glad to know, so I can walk away. 😭

PS: Thank you all so much for the love, kind words and your support! It has given me the strength to do what I needed to do. So much love, very much appreciated.