If you're into consensual non-monogamy but one of your partners wants to just be monogamous?

I think the right decision would be to let that person go, or else they will probably think of you as a cheater. You have already told them that you just want a casual relationship, but they want more than that. Honestly, if your value are conflicting with that person at the foundation of the relationship, that monogamous person is not going to want to stay with you.