Do you want to live in an open marriage? That's a rhetorical question, but some people actually don't mind that type of arrangement. If you're one of the 90% or more who prefer monogamous marriages, expect monogamy and accept nothing less than monogamy.
"Husband, I will not live in an open marriage. Either you stop all contact with EVILWENCH immediately, agree to a transparency plan so I can feel safe, or you move out. I'll help you pack your things if that's the case."