Men not being able to orgasm or go soft while wearing cock rings in fic is one of my pet peeves. They can absolutely do both, though the ring makes it more difficult and can prolong orgasm as well as making it feel more intesne. I wonder, how on earth do people think you get the solid metal ones off again otherwise? Use a hacksaw?
Though you can indeed get chastity devices that prevent a man getting an erection by basically just hurting lots when you he starts getting hard, but you kind of have to put those on before you start...