I wonder how Christine feels about the Ken situation now that she's no longer a polygamist. He was just expressing his views on polygamy, which were negative. And he has a right to that opinion.
I don't blame Christine for having such a strong reaction in the moment. I think her reaction was a symptom of her own struggle in the marriage. While Meri was secure at that point with the legal marriage and Janelle and R were secure in K's love for them, so the remark about how K should leave three of them didn't feel like a genuine threat.
This overall situation is also ironic because Christine was at the point of not wanting K to stay with her even though she later complained that she didn't get enough time with him over the years. On this trip, K decided that he was only going to stay at the campsites with Janelle and Christine and not include the hotels, where Meri and R were staying, in the rotation. So, this should have been heaven for Christine.