Most Helpful Guy
The main problem is that you will instantly be cooling the water down when it hits the milk, so the coffee will never be as hot. It will be less of an issue with coffee than tea because the heat of the water in tea affects how the bag will allow it to brew. Still, you are likely to end up with a less than hot cup of coffee.
Most Helpful Girl
Coffee has to be made with water. It doesn't really matter if you pour milk in the cup before or after you pour the coffee.