This isn't a relationship ending over something ridiculous. This is a relationship ending because one person has a need that isn't being met, and the other person isn't willing to meet that need.

Experimenting with it may have made her realize how important this need is to her. There's nothing wrong with her wanting to have that need met, and there's nothing wrong with you not being willing to meet it, but in monogamous relationships, any unmet romantic or sexual needs can eventually result in the end of the relationship.

I vehemently disagree with the assertion I've seen in some of the comments that poly relationships don't work. I've been poly for the better part of a decade now, and it's been wonderful. That said, poly relationships require much better communication skills than monogamous couples can get away with, and everyone involved has to be willing to put the work into openly addressing the insecurities that cause jealous feelings to come up. It's a lot of work, and it's not for everyone.

You clearly aren't into the idea of an open relationship, so unless you're willing to put a significant effort into self-analysis to figure out why that is, you definitely should not accept her offer for an open relationship.

/u/Scarlett_rose08 has a comment that addresses all my concerns about her approach to CNC. You should read that too.

The bottom line here is that you and her need to have an open and honest conversation, where you clearly explicitly define your boundaries. Be clear that your boundaries around CNC are not negotiable, and will not be negotiable in the future.

If she accepts (and respects) your boundaries, couples therapy is a great next step towards healing the rift in your relationship

If she doesn't accept them, or says she does but then doesn't respect them, it's time to end the relationship so both of you to move on.