It's ironic that two favorite drinks of guys are just plain dangerous to all they hold dear as a male: beer and milk. I've already covered Beer and Testosterone, but now let's look at that white stuff that is squeezed out of a cow's mammaries and see if it really "does a body good."
NOTE: For the record, I love beer and tequila. However, I recognize that the science says that they don't love me.
Dairy is an integral part of the typical American's diet. It's added to everything in one form or another: milk, cream, cheese, ice cream, yogurt, sour cream - the list could go on and on. American, on average, consume about 25, 20 and pounds of milk, cheese and ice cream, respectively per year. That is a LOT of dairy and I'm going to show below that it is likely causing a lot of problems, both below the belt and above.
Here are five great reasons you should set down that glass of milk:
1. Calcitrol. Most dairy products proudly discuss how much calcium they contain. Unfortunately, it is easy for a dairy lover to actually get too much calcium, leading to lower calcitrol levels. This, in turn, signficantly increases his risk for prostate cancer.  For more information, see this link on Prostate Cancer.
2. Estrogens and Cancer. You mama weaned you off of breast milk a long time ago and for good reason: her breast milk is chock full of estrogens. Well, so is cow's milk and researchers have long noted that many types of hormone-related cancers, such as breast, testicular and prostate, are significantly higher in populations with high levels of milk and dairy consumption. 
One of the issues that affects estrogen levels in milk is the cow's stage of pregnancy.  Perhaps regulation can help one day with this issue, but in the meantime, a glass of milk may give you a lot more than you bargained for. Again, isn't it ironic that two favorite beverages for men, milk and beer, are under fire for their estrogen-enhancing properties?
3. Inflammation. Cassein - the primary protein in milk - decreases the body's ability to use anti-inflammatory phytochemicals in food and is likely pro-inflammatory in and of itself. 
4. Cancer. Milk increases IGF-1 and may, therefore, put one at risk for many non-hormonal types of cancer, such as colon. UPDATE: I found many other important links and placed them in my page on Milk and Cancer. There I show that milk has growth factors linked to cancer, a gut inflaming protein and a decreasing effect on the body's own anti-cancer molecule (calcitrol).
5. Heart Disease. There is actually more than one type of cassein molecule. One of them, beta casein A1, has been linked to heart disease from epidemiological observations and several animal studies. The reason? It breaks down in the blood stream and causes inflammation in arterial walls.  One 2003 animal study showed that Beta casein A1 led to increased arterial lesions (damage) and thus would like lead to increased arterial plaque buildup.  NOTE: One human study did not show any such correlation, so follow up work needs to be done. 
WHEY: Most of the above comments do not apply to moderate consumption of whey, which has, for example, reasonable levels of calcium. For more information, read this link on Undenatured Whey.
YOUR WOMAN: Many women consume lots of dairy in the hopes of preserving bone health. Here's "food for thought": in almost all of the supercultures of the world, dairy is used only minimally and yet there is a muchreduced rate of osteoporosis amoung women. Studies of the Okinawans reveal that the divergence occurs during middle age, i.e. at middle age those on a Western Diet begin to experience accelerated rates of bone loss.  Scientists are unsure exactly why, but it is likely a combination of the 1) high magnesium, 2) abundant exercise and the 3) low animal and dairy product consumption.
In the meantime, your woman's consumption of dairy may be increasing her risk of breast cancer and uterine fibroids. Uterine fibroids are benign tumors that afflict almost all women. However, some experts think that dairy, due to the elevated estrogens, accelerate fibroid growth. When fibroids become large enough, they can cause heavy menstrual bleeding, severe cramping and painful intercourse. Yes, that means you have a miserable woman who is less interested in sex.
MYTH: One belief about milk that is probably largely myth is its "mucus producing" properties. There is little to no evidence that milk promotes asthma, allergies or mucus, except of course in sensitive individuals with a true food allergy.  Of course, milk is one of the most common food allergies and may be as high as 8% of the population. 
1) Cancer Epidemiol Biomarkers Prev,2006,15(2):203-10
4) J of Isfahan Med School, 25(87), "Association between Dairy Consumption and Circulating Levels of Inflammatory Markers among Women"
6) Atherosclerosis, 2003 Sep, 170(1):13-9, "A casein variant in cow's milk is atherogenic"
7) Br J Nutr, 2006 Jan, 95(1):136-44, "Effect of dietary supplementation with beta-casein A1 or A2 on markers of disease development in individuals at high risk of cardiovascular disease"
8) Domest Anim Endocrinol, 2002 Jul, 23(1-2):125-37, "Mammary secretion of oestrogens in the cow"
9) J Am Coll Nutr December 2005, (24suppl)(6)547S-555S, "Milk Consumption Does Not Lead to Mucus Production or Occurrence of Asthma"
10) Journal of Allergy and Clinical Immunology, July 2001, 108(1):133-140, "Population study of food allergy in France"