I think some of the confusion is coming from the fact that not only do Jack and alexandraerin have a D/s relationship, but also due to Jack and Kynn also having a safeword in their relationship. For a lot of people, safewords are distinctly associated with kink.

But what it comes down to in the end is that it doesn't matter. It doesn't matter if Jack was Kynn's onlt partner or if he had dozens of others. It doesn't matter if they were as vanilla as possible or the kinkiest couple ever or anything else. Nothing in that spectrum excuses what she did.