Many people find the beer-and-milk combination pleasant when done correctly, like in a beer nog or Guinness milkshake.
Done incorrectly, the beer will curdle the milk or make it difficult for your body to absorb the milk, leading to diarrhea. Milk stouts do not curdle because the lactose lends creaminess, but there is no actual milk used.
Combining beer and milk as a flavor is up to you and your taste buds. Objectively, however, there are better mixes out there that aren’t as likely to curdle. Either way, it is a mix you can drink as long as it’s not in excess.
More than likely, the beer will make the milk curdle. The fats that protect the proteins in milk aren’t there in 2% milk. The acidity of the beer will cause the unprotected proteins to clump up and begin curdling.
Beer can still curdle milk in your stomach. The same reaction happens with the same components. However, milk will curdle even without a beer in your stomach because of your stomach acid.
Drinking milk after a couple of beers is worse than drinking milk before beer. That said, as long as it’s just a glass, it shouldn’t affect you any more than the beer will if you drink too much.
You can drink a glass of milk before beer with no unfortunate side effects. This can be done a couple of hours or just before drinking beer.
Beer and milk cocktails do exist, but they aren’t very common due to the fact that it is not a common palate that enjoys milk and beer together.