Yes, milk causes osteoporosis. Britney gave the best answer so far. It is well documented by many scientific studies.
You do need calcium, but not from animal sources. Animal protein metabolizes in our bodies forming two strong acids, sulfuric and phosphoric. To keep our blood pH slightly alkaline, our bodies need to neutralize (buffer) these acids, and calcium is the best substance our bodies have to do this. And the most plentiful source of calcium is, you guessed it, our bones. Animal protein also contains large amounts of phosphorous, which reacts with calcium to form an insoluble compound, inhibiting calcium absorption. If you eat a diet high in animal protein, it doesn't matter how much calcium you consume, you will still lose bone mass.
While soy or plant proteins do not leach your bone calcium and allow you to maintain a positive calcium balance in your body.
[Breslau NA, et al. Relationship of animal protein-rich diet to kidney stone formation and calcium metabolism. J Clin Endocrinol Metab 1988; 66(1):140-6. ]
Read for yourself here: http://www.babyreference.com/MilkingYourBones.htm
Milk contains a cocktail of harmful hormones, allergens, antibiotics, bad fats, rancid cholesterol, toxins, cow blood, bacteria, viruses, PCB's, dioxins, heavy metals, and more.
I know I feel much better since I stopped drinking it 3 years ago.