If a mother chooses to breast feed, she willingly feeds the milk that her body produces to her child. That’s a positive and consensual process.
A primary reason that people think to question whether breast milk is vegan is because they see a parallel between drinking your human mother’s milk and drinking a cow mother’s milk. But those two things are radically different.
Unlike the breastfeeding situation described above, drinking a cow’s milk is not a positive or consensual process.
There’s no way around it: female cows are forcibly inseminated, have their newborns taken away from them, and then have their milk removed to be fed to someone else.
If we obtained breast milk by forcibly inseminating women, taking their newborns away, and then forcibly removing their breast milk to feed to someone else, then no, I wouldn’t be ok with breast milk, either. I think treating women this way is horrifying and unacceptable, whether they’re human women or female cows.
The confusion here seems to stem from the misconception that milk is naturally or peacefully collected from cows, when this is actually very far from the case. For more on this, check out How could you stop eating CHEESE?!