Come on, guys, it's milk...
It's like asking "uhhh, will cake make you fat?" and responding with "you are unable to digest the fats and carbohydrates, and there are not enough enzymes in the chocolate mousse, etc."
The answer is simpler: just look at the F'ing nutritional information
. There are about 10-12 grams of sugar and 3-7 grams of fat in each cup of milk. If you chug 45 gallons a day of the stuff, you'll be consuming lots of sugar and fat. Of course you'll gain some weight, and it'll be bad for you. It's not because of the mucus, or blood, or this or that, etc.
Some people have sworn it causes acne, or bloating...whatever, it's genetics for some people. But with most people, a few glasses a day provides a good source of protein, calcium, and fats.