For your third suggestion, I'll take a shot: For animals to lactate, they must become pregnant. Dairy cows are repeatedly impregnated, they give birth, and they are separated from their calves. Female calves become dairy cows themselves, male calves are either made into veal or killed promptly. A small fraction may become studs for more dairy calves.

The entire ordeal is very much not natural and not pleasant. And we do it at a scale that is awe-inducing. If you are a compassionate type that sees cows in a similar light to dogs, I'd bet you can understand why someone would feel very strongly that it's not an option.

If you would like to learn more there are a few documentaries that explore the big 3 benefits of veganism--Health, ecology, and animal welfare. I won't link them here as they are easy to find for any that are interested.