Just imagine it. You get your girlfriend to lock your penis in a chastity cage for one month. Then you still have to keep on pleasing her. Kissing her, sucking her tits, going down on her etc. If she wants penile penetration you would have to wear a strap on. Worse still, she could chain you to the bed and tease you all night long while you're still in your cage, the whole time, unable to get hard or even masturbate afterwards. Imagine the frustration. And just imagine the releif and explosive orgasm you would have the night she finally unlocks you!
This thought just makes me horney as hell. And even when she does unlock you, she might decide one night is enough and locks away your penis for yet another month!