Right now Paleo and Low Carb diets are BY FAR the most popular here in the U.S. among men from all that I have seen. They simply dominate the health and fitness landscape and vegetarianism is now associated with weakness and ill health. This is the ultimate irony, however: vegetarians actually enjoy superior longevity and significantly increased mortality rates.
Now why would the health and fitness community not promote the dietary regimen that produces the greatest longevity? Well, I don't know either, but I want to start by pointing out a common theme: vegetarians have improved longevity almost entirely due to reduced cardiovascular mortality rates. In other words, vegetarians generally live longer simply because heart disease is the number one killer and vegetarianism greatly reduces the risk of dying from heart disease. There are many reasons for this, but here is something that I want to point out immediately: anything that improves your cardiovascular system is VERY likely to improve your sexual health and erectile strength.
The reason is that the cardiovascular and penile/erectile systems are joined at the hip so to speak. Erections are very dependent on blood flow and clean arteries and so is heart health as well. This is one of the reasons I am largely vegetarian: it's G-O-O-D for the bedroom.
Anyway, here are just some of the studies that show improved mortality and longevity rates from a vegetarian lifestyle:
1. Meta-Analysis. One journal article pooled five studies and found that vegetarians had significantly reduced cardiovascular mortality rates, i.e. the risk of dying from heart-related issues.  Those who ate fish but not meat or were lactoovovegetarians enjoyed a particularly large benefit and decreased their death rates by a little over a third.
The abstract shows has one and only one sentence in it: "Vegetarians have a lower risk of dying from ischaemic heart disease than non-vegetarians."
NOTE: I consider myself a "Plant-Based Eater". In my mind, this has a less dogmatic connotation that avoids a lot of the unnecessary rhetoric that sometimes goes along with being a vegetarian. For example, I do eat occasionally eat meat and consume eggs almost daily. I also am willing to admit some of the advantages that eating meat confers. But, overall, in my mind vegetarianism is the clear winner overall.
2. Large Adventist Health Study. This very large study of over 34,000 Seventh Day Adventists (SDAs) shows very similar results to #1: vegetarian men enjoy improved cardiovascular mortality rates by about one third or even a little over.  (fyi: They also found that vegetaarian Adventists had lower risks of diabetes, arthritis and high blood pressure than meat-eating Adventists.
3. The Other China Study. This study from China made a mishmash of the research by including some semi-vegetarian groups in it. Due to not making careful distinctions between vegetarians and non-vegetarians, conclusions were hard to come by. Still the researchers will willing to state clearly: "The mortality from ischemic heart disease was significantly lower in vegetarians than in nonvegetarians (RR = 0.71; 95% CI, 0.56 0.87)." 
4. European Vegetarians.  This research summary out of Slovakia was clearly vege-skeptic. For example, the authors noted that vegetarianism conferred little benefit in the cancer arena, but admitted that "in vegetarians, a decrease of ischemic heart disease mortality was observed probably due to lower total serum cholesterol levels, lower prevalence of obesity and higher consumption of antioxidants. Very probably, an ample consumption of fruits and vegetables and not the exclusion of meat make vegetarians healthful." This is the ultimate irony: the researchers try to defend meat eating and yet admit that fruits and vegetables and high antioxidants (obviously from plants) are keys to cardiovascular improvements that beat meat out every time. So what's not to like??
The summary of all this research data can be summarized as follows:
1. Several studies show big benefits in longevity to vegetarians. These mortality benefits are largely cardiovascular.
2. No studies (that I know of) show any significant advantage to eating meat in terms of extending life span or reducing death rates.
3. A couple of studies show relatively neutral benefits in terms of all cause mortality and cancer.
4. All studies show vegetarians experience an improvement in ischemic heart disease mortality rates. We will discuss this more below.
Now a quick side comment: in light of this data, the evidence points to vegetarianism as being the likely winner. Admittedly, the data is not as consistent as one would like. But meat never wins and has often lost.
In light of this, one cannot help but ask, "Why do herbivores (meat-eaters) so aggressive attack vegetarian diets?" It simply does not make sense based on the data. Again, just go to the Paleo and Low Carb sites and you will constant charges that a) vegetarian diets will increase heart disease (in spite of the evidence), b) are packed with "anti-nutrients" and c) will hammer your testosterone. Yet the evidence actually shows that Vegetarians Are Equal or Better in Testosterone Than Meat Eaters.
But now let's go back to what is probably the most important point about vegetarian diets and longevity: the improvement in ischemic heart disease. This is should be of interest to men everywhere, because ischemic means a "lack of blood supply" or "insufficient supply of blood" to a tissue or organ. In other words, vegetarianism improves blood flow. (Yes, that is what my book, The Peak Erectile Strength Diet is all about.) And, if you're interested in sex, it's all about blood flow. So, again, a vegetarian diet is one of the keys, in my opinion, of becoming a true Orgasmatarian and this is shown clearly in virtually all of these mortality studies.
In addition, I have to point out that I think it is remarkable, actually, that vegetarian diets do so well considering there are so many vegetarains that eat a lot of wheat and other refined carbs. I am particularly afraid of wheat, because it is heavily GMO'd, has a high glycemic index and produces an immune response in many people, a fact that I cover in my link on A Review of Wheat Belly.
It is quite easy to supercharge a vegetarian diet, by limiting things like wheat and refined carbs, and make it even much more healthy and likely to produce superior longevity. This is shown by the fact that the most elite cultures on planet earth with no heart disease, hypertension, cancer, etc. are all almost entirely vegetarian. For more information, see my link on A Review of Healthy at 100.
1) Bratisl Lek Listy, 2008, 109(10):463-6, "Vegetarian diets, chronic diseases and longevity"
2) Am J Clin Nutr, 1999 Sep, 70(3 Suppl):516S-524S, "Mortality in vegetarians and nonvegetarians: detailed findings from a collaborative analysis of 5 prospective studies"
3) Am J Clin Nutr, 1999 Sep, 70(3 Suppl):532S-538S, "Associations between diet and cancer, ischemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists"
4) Ann Nutr Metab, 2012, 60:233-240, "Cardiovascular Disease Mortality and Cancer Incidence in Vegetarians: A Meta-Analysis and Systematic Review"