What is the number one thing most men can do to boost their sex life? Well, exercise is right up there along with sleep and stress management. However, the king in this category is undoubtedly plant-based nutrition for many reasons. One huge tragedy in most modern societies is that meat is synonymous with manliness. And, for this reason in my opinion, meat-based lifestyles (Low Carb, Atkins, the high fat Americanized version of Paleo, etc.) are incredibly popular, much moreso than any other dietary regimen here in the U.S. Again, though, this is ironic, because guys are all about sex and impressing their woman and it is plants that are going to help them most in this category.
Yes, plant-based nutrition should be called the True Manly Diet in my opinion and, hopefully, below I can give you many benefits and reasons just why this is so. I myself am a lacto ovo vegetarian, meaning I don't eat meat but do eat egg whites and non-fat yogurt to decrease carbohydrates and for additional protein. (I'm into buidling and maintaining muscle so want extra protein sources.)
By this definition I would be "plant-based" since I get over 90% of my calories from plants. However, the general definition that I see out there for plant-based is basically 100% vegan or plant foods but ONLY whole plant foods. Plant-based vegan pride themselves, as well they should, on only eating whole natural plant foods.
Regardless of what label you put on yourself, here are 8 Huge Benefits of Going On 90+% Plant-Based Diet or Even a True Whole Foods 100% Plant-Based Diet::
1. Blood Flow. Many plant-based foods will lower blood pressure. If you'll notice in my link on How to Lower Blood Pressure, there are probably a dozen plants that have been shown in the research to lower blood pressure. Of course, this is very important, because high blood pressure is a risk factor for erectile dysfunction, stroke and heart disease for starters. (It is also part of the modern plague called the Metabolic Sydrome that cripples so many men in middle age and beyond.)
NOTE: Personally, I would avoid wheat for reasons I outline in my Review of Wheat Belly by Dr. William Davis. I would also avoid corn, because it is highly GMO'd with a pesticide called BT toxin (at least here in the U.S.).
2. Testosterone. There are indications that plant-based systems of eating tend to have testosterone levels as high or higher than their meat-eating equivalents. One study showed that vegetarians had testosterone 6% higher than their meat-eating counterparts. (See my link on Testosterone and Vegetarianism for more information.) And what will be really counterintuitve for many men is that vegans - vegetarians that do not eat any dairy or eggs - had 16% higher testosterone than the equivalent carnivores in the study. And the Okinawans, who eat only small amounts of meat, maintain high testosterone levels decades part the typical man in a Western society.
Yes, gone are the old myths that meat eaters are the only ones with decent testosterone. It appears that the opposite may be the case. And it makes sense: many of the more vegetarian, plant-based diets emphasize monounsaturated fats, which are known to be pro-testosterone. Furthermore, these types of diets are high in antioxidants and anti-inflammatory phytochemicals that will tend to act in a protective manner against the age-related losses of testosterone.
3. Nitric Oxide. It's no secret that all the big nitric oxide-boosting foods and drinks are plants. This is one of the primary subjects of my book, The Peak Erectile Strength Diet.
4. Mortality. Wanna stay alive? A plant-based lifestyle has been shown to do just that. Multiple mortality studies on vegetarians have shown improved mortality rates. What this means is that those who eat vegetarain generally die less frequently. See my link on Mortality Rates for additional information.
5. Change Your Genes. Have "bad genes"? Fruits and vegetables have been shown to overcome this in some cases. For example, one massive study found that ample servings of fruits and vegetables overcame the negative effects of the 9p21 gene that strongly influenes cardiovascular outcomes. 
6. Inflammation Protection. The number one thing that you want your natural lifestyle to do is fight inflammation. Why? Because inflammation is the Root of All Evil. It is the root cause for most of our heart disease, diabetes, autoimmune disease, cancer and erectile dysfunction. And, yes, many studies have shown that fruits and vegetables dramatically lower inflmmation. For example, a 2003 study that in just one month participants who ate a diet high in fruits, vegetables and nuts lowered this C-Reactive Protein levels by 28%.  Yet another study found that eating fruit lowered CRP levels by about 20% from lowest to highest consumption and vegetables by almost 30%.  Not bad for just one component of ones diet! Studies on just healthy males alone have shown the same thing.  For additional, read my link on How to Lower Inflammation and Inflammation and Juice.
7. Metabolic Syndrome. Fruit and vegetable consumption is associated with decreased risk for developing the epidemic plague in modern societies called The Metabolic Syndrome.  Both fruits and vegetable consumption was tied to a reduction in risk of about a third.
8. What About Grains?? One bias that many men have is against whole grains due to the prolieration of and misinformation from many Low Carb and Paleo sites of late. This is unfortunate because WHOLE grains are every bit as outstanding for health as fruits and vegetables. Common sense tells you this, because grains are a substantial component of many of the healthiest diets around the world including the Mediterranean, the cultures in Healthy at 100 and the Tarahumara.
As I show in my link on Grains and Your Penis, whole grains do everything you could possibly want as a man, lowering blood pressure, increasing nitric oxide and flooding your system with anti-inflammatory and antioxidant compounds. It's literally a dream-come-true.
Of course, the key is that grains are only healthy if they are whole grains. I also caution against too much wheat consumption, whole or not, due to its heavy GMO modifications in the last fifty years. See my Review of Wheat Belly for details.
2) Am J Clin Nutr, Dec 2006, 84(6)1489-1497, "Fruit intake vegetable intake metabolic syndrome cardiovascular disease risk factors inflammation"
4) Am J Clin Nutr, Nov 2005, 82(5):1052-1058, "A 4-wk intervention with high intake of carotenoid-rich vegetables and fruit reduces plasma C-reactive protein in healthy, nonsmoking men"