Introduce your girlfriend to the joy of the ruined orgasm.
Slide it out of her just as she cums. It’ll keep her horny and desperate for more.
Something that you’ll both come to love, and crave.
Have you tried shoving your juice covered cock in her mouth right as you pull out and her orgasm is ruined? Make her suck while she misses the orgasm, and see how she reacts. You’ll like it.
I’d be slightly worried about her getting bitey in her frustration, but I do like the way you think.
Better safe than sorry, get out the ring gag.