R3 Defend your R1 argument against opponent's R2 rebuttal
Bad to the bone = immoral.
Full resolution: Drinking cow's milk is usually immoral.
Exceptions, people with rare medical conditions, and extraordinary circumstances. For example, stranded on a desert island and/or about to go into a diabetic coma.
Burden of proof will be on me to prove that drinking cow's milk is usually immoral.
II. Animal ethics
We shouldn't drink bovine milk because it encourages the slavery, exploitation, and mistreatment of cows. Not only that, but slavery, exploitation, and mistreatment of one animal paves the way for this same treatment towards other animals. Cows milk is unhealthy nor environmentally sustainable.
II. Animal ethics
"A factory farm is a large, industrial operation that raises large numbers of animals for food. Over 99% of farm animals in the U.S. are raised in factory farms, which focus on profit and efficiency at the expense of animal welfare." 
Animals on so called "humane" farms don't fare much better. 
"Even under the best circumstances, the incentive to treat animals “humanely” is limited to the extent to which it is necessary to raise them to market weight (which is just a fraction of their natural lifespan). Any humane practice beyond this would be seen as inefficient and unsustainable by today’s business standards." 
Between the inherit conflict of interest between animal welfare and profit. and the brutal realities there is no moral justification for humans drinking bovine milk.
Milk causes osteoporosis and increases chances of hip fractures. Contains, IGF-1, cholesterol, casein, lactose, high in fat, high in saturated fat, and high in dead bacteria endotoxins.
"These results support the hypothesis that dairy products and calcium are associated with a greater risk of prostate cancer. " 
" Studies have shown that IGFs are potent mitogens for a variety of cancer cells including prostate cancer since they stimulate cancer cell growth and suppress programmed cell death. " 
"Naturally occurring milk IGF-1 levels were recorded in 5777 random milk samples from the Bavarian dairy cow population." 
As you can see from above, milk increases cancer risk. IGF-1 is found in cow's milk and is associated with increased cancer risk.
"These findings suggest that even small intakes of foods of animal origin are associated with significant increases in plasma cholesterol concentrations, which are associated, in turn, with significant increases in chronic degenerative disease mortality rates." 
Casein is a protein that can cause an immune system response which can lead to type I diabetes.
“Total protein consumption did not correlate with diabetes incidence (r = +0.402), but consumption of the beta-casein A1 variant did (r = +0.726). Even more pronounced was the relation between beta-casein (A1+B) consumption and diabetes (r = +0.982). These latter two cow caseins yield a bioactive peptide beta-casomorphin-7 after in vitro digestion with intestinal enzymes whereas the common A2 variant or the corresponding human or goat caseins do not. beta-casomorphin-7 has opioid properties including immunosuppression, which could account for the specificity of the relation between the consumption of some but not all beta-casein variants and diabetes incidence. “ 
Women who drank three glasses of milk or more every day had a nearly doubled risk of death and cardiovascular disease, and a 44 percent increased risk of cancer compared to women who drank less than one glass per day, the researchers found.
Men's overall risk of death increased about 10 percent when they drank three or more glasses of milk daily, said the study, published online Oct. 28 in BMJ."” 
“The risk of any bone fracture increased 16 percent in women who drank three or more glasses daily, and the risk of a broken hip increased 60 percent, the findings indicated. “ 
Fat is not only the densest of the three macro nutrients at 9 calories per gram as opposed to 4 for carbohydrates and proteins. This can lead to obesity and intramyocellular lipid build up.
Saturated fat increases blood viscosity which makes the heart work harder. Lactose is a sugar that many people are intolerant towards.
Inherently due to trophic levels milk must be worse for the environment. From the American Journal of Clinical nutrition we can see that lupine, a vegan alternative, is better for the environment than cow's milk. 
In conclusion, there is only one ethical choice, and that is to stop drinking bovine milk. Thanks for the debate.