If this is a deal-breaker for you, the only possible way to change her mind is to tell her exactly that - and then be prepared to follow through. It's probably not too soon for you to lawyer up and start talking about divorce and child custody, if she is determined to go through with this terrible plan.

If you married with the assumption of monogamy, you absolutely have a right to insist that she remains monogamous if she wants to stay married to you. She voluntarily chose to limit her own sexuality and restrict access to her body, when she took her marriage vows. Don't let her try to accuse you of anything more than reminding her she's married.