I think Shun appeasing tamaki's trauma by psychologically showing he's trying to own the blame of their sexual intimacy makes it easier for tamaki to accept his advances little by little in baby steps pace so they could build a relationship of trust leading to tamaki being able to breakthrough that will lead to a good foundation to be able to start having a normal lovers skinship without the baggage of tamaki's trauma weighing them down.
I think Shun appeasing tamaki's trauma by psychologically showing he's trying to own the blame of their sexual intimacy makes it easier for tamaki to accept his advances little by little in baby steps pace so they could build a relationship of trust leading to tamaki being able to breakthrough that will lead to a good foundation to be able to start having a normal lovers skinship without the baggage of tamaki's trauma weighing them down.