I wonder if the memories bring comfort because you know you survived it, but you still get the fear rush to enhance the sex which is occurring in a very safe trusting relationship. You are doing what so many survivors of abuse do, and recreating the trauma. Most do it and have the same bad outcome. But you have done it and created a good outcome -- Very therapeutic!

With the vanilla sex, it is too close to the original trauma. It doesn't require the same level of trust and involvement. With BDSM and age play you are actively accepting the events so have control, like Euryleia said. The vanilla is too passive so it brings you closer to that feeling of lack of control and powerlessness and triggers the negative reaction.

That is all just a wild guess on my part, but it might explain your reactions if it rings true for you.