Tell her how beautiful she looks & how sexy & intimate it is to create your very own abstract art , using her as your canvas. make her understand that your cum is all you , your most intimate inner offering & how great she looks when you can share it with her like that. Tell her it's the only thing that can make her more beautiful in your eyes. If she understands that you appreciate how beautiful she looks, then she will start FEELING more beautiful & will look at it as a shared sensual experience, instead of a chore , or having you feel like you got lucky or owe her one. Once you master that , then she may come to think of herself as a canvas, & might be open to other stuff in the future. It's a long road , stick with it ! But you are definitely not alone in that fantasy.