This may be a myth only for inept men. The G-spot is a mound of super-sensitive spongelike tissue located within the roof of the vagina, just inside the entrance. Proper stimulation of the G-spot can produce intense orgasms. Because of its difficult to reach location and the fact that it is most successfully stimulated manually, the G-spot is not routinely activated for most women during vaginal intercourse. While this has led some skeptics to doubt its existence, research has demonstrated that a different sort of tissue does exist in this location. You can find more about man orgasm here https://blog.uberhorny.com/what-men-can-do-to-make-their-orgasms-even-better/, but you should find something more about G-spot to sutisfy your girlfriend.
