We sell breast milk without forcefully inseminating women just fine, this leap of logic doesn't make any sense. We treat cows as livestock because we view cows as animals not people, whether their treatment is cruel or not is a different irrelevant argument, but in Zootopia they'd just be seen as people and would have control over the supply so obviously cows wouldn't be treated the same way in Zootopia as they are in real life. They'd only have to milk themselves and sell it just like when women sell breast milk in real life. If anything it's even less weird and disturbing to think about than in real life.

In Zootopia it's probably a job they get compensated for.

Tell me you know nothing about farming without actually saying it.

Cows and other livestock are raised in farms to raise more cows anyway. Farmers don't keep male cows around for fun, and cows aren't pets. All non-breeding stock gets eaten.

Your last paragraph is part of the premise of Mad Max: Fury Road

I'm not sure about other animals, but humans can induce lactation without pregnancy.

don't compare women to cows

The milk drinking is just as weird, since no other animal continues drinking milk after passing their baby state. Only humans do that.

How is it more weird to get milk from intelligent animals that are capable of giving consent?

Humans sell their own breastmilk to other humans for human consumption all the time in real life, it is completely normal for humans to drink human milk as well as the milk or other animals. The only difference in Zootopia is that every animal would be providing their milk consensually and likely selling it themselves. There's nothing weird about that.