A whole foods, plant-based diet is a healthy diet. This means that a vegan person is more healthy. Health renders a state of well-being. Well-being is better than non-well-being. Thus, a vegan diet is more ethical when it pertains to humans.
A global shift towards a vegan diet is necessary not only to ease the suffering of the millions of animals killed daily, but also to save the world from hunger, fuel poverty and the worst impacts of climate change.
Many people cannot afford or do not have access to vegan foods. It would be unethical to establish a system of ethics that uses money or circumstance as a barrier to entry.
This assumes you care about other animals. If you do not care then it is not more ethical to not eat them than eat it.
Animals in nature eat each other as a way of life. Limiting human's nature as omnivores is unethical.
http://abcnews.go.com/Technology/story?id=97836 And many more sources.
Some animals that have been bred to produce milk for human consumption rely on being milked regularly or they will otherwise suffer pain and eventually might die. Completely and abruptly switching to a vegan lifestyle would therefore cause animals to suffer.
once asked a dairy farmer about this and he said that (abrupt) stopping of milking will likely cause inflammations and other complications
Ethics are a construct created by humans. Nature is not ethical. Survival is natural. The strong survive, and the strong need protein. Long live steak.
I am an omnivore.