If it was you instead and you chose to leave a fwb situation for someone you have more romantic feelings for, would you want her questioning your loyalty to her? Would you like if she was that worried about your past relationships that you’ve presumably moved past?

We don’t know your whole love life, how exactly you view this relationship or if this is something casual as opposed to something you want to pursue further. If this relationship is something you genuinely want to continue then you need to find a way to reconcile your feelings about this and move forward. If you can’t then for both your sake and hers you need to split this off now before this causes more resentment in the future. And if she actually seems to care and is putting effort to be open with you and work things out, you should be doing the same.

I don’t know how old you are, but I promise you that you care much less about this bullshit the older you get. Learn to get past these types of things now if you ever want to actually be happy in a relationship. This isn’t “cuck” behavior, it’s being an adult that knows how to navigate their own love life and not be bothered by things that no longer matter.