I agree with the other posters, it is not dangerous.
Although not sure about the vagina, but if the posters meant it is safe with both the butt and the vagina, then it is safe.
Also remember, the ice cube you are probably thinking of, there is not a lot of water that makes it up, that won't be enough to bring down your body temperature too quickly. Also, in between the melted ice cube and the next ice cube, your butt would already have started to warm. If you are in a warm room, then that helps to warm your butt quickly.
But, don't use what I just said to base your judgment on, I encourage you to consider it yes, but remember, it all comes down to what you feel comfortable with.