"Don't be a soy boy!" You may have heard that expression if you've hung around any of the bodybuilding sites. Is it true? Is soy bad for us guys?
In my opinion, soy is one of the most complex of all foods to evaluate, because it is like us: when it is good, iti is very, very good; and, when it is bad, it is very, very bad. Yes, it is truly both angel AND devil.
Let's start with some of soy's incredible properties that can help us males. Here's just a few that should raise your eyebrows:
1. Nitric Oxide. A fairly recent animal study showed soy increasing vascular reactivity, i.e. arterial "responsiveness", and lowering blood pressure.  Furthermore, one study on men (and women) with mild to moderate high blood pressure showed a very dramatic reduction in blood pressure after several months of soy milk consumption.  Results were striking and showed a 18.4 and 15.9 mm average decrease in systolic and diastolic pressures, respectively. This is because soy affects eNOS favorably and can result in increased nitric oxide. One in vitro study on human endothelial cells found a 70% increase in nitric oxide from genistein. 
2. Okinawans. One of the best documented supercultures on the planet are the Okinawans. They enjoyed incredibly good health and longevity in spite of living in a modern, industrial environment and are famed for their high centennarian and low heart disease and cancer rates. The Okinawans ate considerable amounts of soy, probably an average of 25-50 grams daily.
3. Cardiovascular Disease. Soy is associated with decreased heart disease in both men and women. One of the reasons, besides its blood pressure lowering effects, is it also decreases cholesterol in men with high cholesterol levels.  It also lowers homocysteine and oxidized LDL cholesterol levels, both significant risk factors for cardiovascular disease.  Furthermore, it also likely lowers saturated fat consumption for many, simply because it is (for those who like it) a satisfying protein food that lowers dependency on meat and dairy. Later study work partially verified similar results on cholesterol, by the way, although results were not uniform. 
4. All Cause and Cardiovascular Mortality. One study on Japanese men showed a 23% reduction in all cause mortality for men who consumed soy 4+ times/week compared to those who consumed it once per month.  That's a pretty impressive reduction and cardiovascular mortality was reduced as well, which is the #1 killer of men in modern societies.
5. Quality, Undenatured Protein. Soy is also a good quality protein, in spite of what some bodybuilding sites claim, and is usually inexpensive and easy to obtain. Even more attractive is the fact that there are now many undenatured brands available both online and in many health food stores. It has become a staple for many vegan and vegetarian bodybuilders because of it.
6. Prostate Cancer. One of the big worries for middle-aged and older men is prostate cancer. I just had a friend die of it: after months of chemo, it still spread to his liver. Several studies have shown that soy is protects significantly against prostate cancer. 
7. Non-alcoholic Fatty Liver Disease (NALFD). Preliminary studies seem to show that soy reduces fatty liver disease in those who are overweight at least and helps stop the accumulation of fat in the liver. 
Of course, these benefits are extremely impressive and some researchers would argue that soy is worth it just for the cardiovascular protective benefits: after all that kills more men and fills up more hospitals than any other single factor in our society.
In spite of all these stellar benefits, I am still very cautious about soy and rarely consume it. As it turns out, the dangers and risks are just as dramatic:
1. Hypothyroidism. Hypothyroidism, or an underactive thyroid, is a risk factor for low testosterone and can affect erections and relationships as well. The fatigue and weight gain associated with it are not going to help a man in or out of the bedroom. Unfortunately, soy is a powerful inhibitor of a key thyroid enzyme named TPO that liberates iodine to do its job.  On the surface, it would seem likely that this inhibition can, in some individuals, lead to hypothyroidism, especially in those with low iodine levels (which is quite common). In addition, many men already have decreased thyroid function and additional soy in the diet may exacerbate this condition as well. It is also scary that researchers believe that this effect on the thyroid is due primarily to the phytoestrogens present n soy. These phytoestrogens are quite good at mimicking estradiol, a known promoter of thyroid cancer as well. 
Are these concerns overblown? One prominent meta-analysis thinks so: they looked at multiple studies and concluded that soy had minimal affect on individuals with any kind of normal thyroid function.  In fact, there summary was: "Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function." And they did admit there was evidence that thyroid patients might have to increase their dosage due to the effects of soy consumption.
2. Zinc Chelation. Soy is packed with heat-insenstive phytic acid. Phytic acid has a beneficial propertity in that it lowers cholesterol. However, one very negative aspect of phytic acid consumption is its uncanny ability to bind to zinc.  (It also binds to calcium and iron.) Zinc is critical for testosterone production, estrogen de-aromatization and many other key male pathways. Plus, sexually active men will lose a significant amoutn of zinc in sperm. For more details, see my page on Zinc and ZMA and my new book, 15 Natural Erectile Dysfunction Cures.
3. Male Brain Shrinkage. This is the biggee for me. A couple of studies show that soy can lead to brain shrinkage in males.  The exact wording in the studies is "brain atrophy" and it only applies to males. Again, this is likely another negative effect of the phytoestrogens in soy.
Now I should point out the counterargument to this: one study shows an improvment in memory in Indonesian seniors who regularly consumed soy.  This is rather hard to explain if soy is really so brain-toxic as is the very-sharp-through-old-age Okinawans. In short, it looks as if these claims against may be somewhat exagerrated and/or need additional studywork. However, the rebuttal to this is that a follow-up study showed that tempeh improved memory in the elderly but tofu decreased it.  So perhaps the difference is fermented versus non - who knows?
4. Low Testosterone and Hypogonadism. Previous study work was somewhat unequivocal but likely showing a slight decrease in testosterone resulting from soy consumption. However, one recent study should give every male pause to reflect: a case report of a male on a high soy diet that developed hypogonadism (clinically low testosterone) and was able to fully recuperate upon cessation. 
5. Hexane. One thing that I don't think anyone disputes is that modern soys are generally processed with hexane, a toxin that has no business being on your plate. And don't let anyone fool you: hexane residues definitely end up on most soy products and many health officials are concerned about it.
In my opinion, soy is impressive but simply not worth the risk, primarily because of risk to the brain and thyroid. The cardiovascular and endothelial benefits can easily be obtained from other foods - see my book The Peak Erectile Strength Diet - coupled with a Low Fat and/or Mediterranean Diet. Yes, it's hard to argue with the Okinawans, but I'd rather be safe than sorry.
1) The FASEB Journal, 2005, 19:1755-1757, "Dietary soy isoflavone induced increases in antioxidant and eNOS gene expression lead to improved endothelial function and reduced blood pressure in vivo"
2) Circulation, 2000, 102:2555-2559, "Soy Protein and Cardiovascular Disease: A Statement for Healthcare Professionals From the Nutrition Committee of the AHA"
3) European Journal of Clinical Nutrition, 2005, 59"843 850, "Effects of soy supplementation on blood lipids and arterial function in hypercholesterolaemic subjects"
4) Preventive Medicine, Jul 2006, 43(1):20-26, "Soy consumption and mortality in Hong Kong: Proxy-reported case control study of all older adult deaths in 1998"
5) J Nutr, Jul 1 2002, 132(7):1900-1902, "Soy Milk Lowers Blood Pressure in Men and Women with Mild to Moderate Essential Hypertension"
6) American Journal of Clinical Nutrition, Aug 2002, 76(2):365-372, "Effects of high- and low-isoflavone soyfoods on blood lipids, oxidized LDL, homocysteine, and blood pressure in hyperlipidemic men and women"
7) Crit Rev Food Sci Nutr, 1995 Nov, 35(6):495-508, "Phytic acid in health and disease"
8) British Journal of Pharmacology, Feb 2005, 144(3):394-399, "The soy isoflavone genistein induces a late but sustained activation of the endothelial nitric oxide-synthase system in vitro"
9) Environ Health Perspect, Jun 2002, 110(Suppl 3):349 353, "Goitrogenic and estrogenic activity of soy isoflavones"
10) J Clin Endocrinol Metab, 2001 Mar, 86(3):1072-7, "Estrogen Promotes Growth of Human Thyroid Tumor Cells by Different Molecular Mechanisms"
11) Thyroid, Mar 29 2006, 16(3), "Thus, collectively the findings provide little evidence that in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function"
12) Brain Research, 16 March 2011, 1379:206 212, "Borobudur revisited: Soy consumption may be associated with better recall in younger, but not in older, rural Indonesian elderly"
13) Dementia and Geriatric Cognition Disorders, 2008, 26(1), "High Tofu Intake Is Associated with Worse Memory in Elderly Indonesian Men and Women"
14) Nutrition, 2011 Jul-Aug, 27(7-8):859-62, "Hypogonadism and erectile dysfunction associated with soy product consumption"
15) Cancer Epidemiol Biomarkers Prev, Jul 2003, 12:665, "Soy and Isoflavone Consumption in Relation to Prostate Cancer Risk in China"
16) Journal of Anti-Aging Medicine, Dec 2003, 6(4):335-336, "Soy-Induced Brain Atrophy?"