Is it rude to start to eat while the cook is still cooking?

As I was preparing dinner, several family members served themselves some food I was still cooking and started to eat. The meal was to be ready in 15 minutes. Instead of finishing the meal, I stopped cooking/preparing dinner. I was offended. Why should I continue to cook the meal to completion if they were already digging in? Were their actions really that rude or did I just overreact? 15 minutes, come on! They could wait! No? Your feedback is appreciated.