Let’s start with the last part.
The answer, of course, is that milk isn’t available in the wild except when it comes from a nursing mother. We humans have an immense production and distribution chain to procure milk, process it to make it safe to drink, transport it, store it, and sell it to us. In our homes, we have refrigeration so that we can store milk safely. When it comes to milk, all of this is what really separates us from adult wild animals, not some divine edict or mystical rule of nature.
Think of that polar bear. Isn’t it better to get your milk from the fridge?
Let’s return to the first part of the original assertion, that drinking milk is harmful to adults. As far as I can tell from a bit of Googling, this is either completely true or utterly false. The “false” position highlights all the healthful nutrients milk contains. The “true” position counters that the same nutrients can be obtained elsewhere and milk also contains lots of saturated fat, which is known to be bad for us.
Don’t beat yourself up about it. Sip a bit of milk. I hear it’s calming and helps fight stress.