Is purchasing factory farmed animal products ethical? If viewed on a practical rather than theoretical level it still means that mass production of animal products would probably be considered unethical by a large section of the population. (If they took the time to look into what happens in factory farming).
If animal products were farmed in what most would consider an ethical way: large areas to roam, no cruelty etcetera then animal products on an "I can eat them every day of the week scale" no longer becomes economically viable. I was looking into this a little and it seems that if enough people were to turn to a plant based diet then the whole factory farming industry would collapse seeing as it is purely an economy of scale. The numbers that have to change aren't even that large. So at this moment in history eating animal products is, in practical terms, synonymous with factory farming and all the cruelty that process entails.
Whichever route you want to take be it animal cruelty, climate change or health, there is, in my opinion, a strong ethical argument for rejecting factory farming of animal products.