Right now high fat diets are very popular, and, unfortunately, if you implement them wrong, you can get yourself into trouble, potentially affecting testosterone and/or erections as I will explain below. But, first of all, how do I definite high fat? A high fat diet is one that gets about 40+% and above of total calories from fat.
NOTE: I am NOT attacking meat eating here, but rather a high fat diet based on modern, industrial meats that are nothing like wild game. Almost all of the problems in this article vanish if one eats range fed meats. I discuss many options in my page on Low Fat Meats.
Most meat sold in the supermarkets here in the U.S. is between 30-50% fat. These are incredibly high fat levels, due to modern livestock methods, with few equivalents in nature. Yes, some fish are an exception, but then they are loaded with omega-3's. Can this possibly be good for a man, especially in the bedroom? I would argue that it is rarely going to help him and I explain why below based on what I am seeing in the research:
1. Too High in Total Fat. One common middle-aged nightmare is insulin resistance. Insulin resistance allows glucose levels to rise, leading to damaged tissues throughout the body and accelerated aging. It also results in a number of metabolic changes that often lead to sexual dysfunction and put a man at risk for many medical conditions such as heart disease, dementia, high blood pressure and diabetes. Another name for insulin resistance is "prediabetes", which I cover here in my link on Metabolic Syndrome and Erectile Dysfunction.
Now what many men do not realize is that a High Fat Diet is so reliable at bringing about insulin resistance that researchers use it in lab animals all the time. In the above link, I document some studies where research have found just how high fat diets do this and it includes alterations to the mitochondria, the powerhouses of your cell which are linked to aging and oxidative damage.
Now will a High Fat Diet always induce insulin resistance in all men? Of course, I cannot say that. But the bottom line is that one is clearly living on the "razor's edge" and should carefully monitor his blood sugar parameters in my opinion.
2. Too High in Saturated Fat. This is something that I cover in great detail elsewhere on the site, such as in my link on The Problems with Saturated Fat. What you should know is that saturated fat slows down blood flow. Saturated fat literally stuns arteries and sludges the cells in the blood disrupting blood flow. This is why young men can usually eat a lot of saturated fat with minimal impact on their sex life, simply because they still have ample nitric oxide to compensate for the negative effects of saturated fat. This is NOT true for many 40+ men, however, and is something to really watch out for. Decreased blood flow negatively impacts many tissues especially, of course, those in the penis and, according to some research, those in the brain.
I find on The Peak Testosterone Forum that few men over abou 45 will argue with me about saturated fat. It's always the young guys pushing the higher fat meals.
3. Too Low in Carbs. A big percentage of men on higher fat diets are actually Low Carb or Ketogenic. There are different variabtions of these diets, but men will consume less than about 50 or 60 grams of carbs per day. These diets have really struggled in the research lately and researchers have found that they signficiantly increase the risk for a) arrhythmias, b) heart damage, c) high LDL-P, d) decreased mood and on and on. You can read about these in my page on The Potential Dangers of a Low Carb Diet for more information. The also greatly limit your abilitity to do higher intensity exercise, something I hope all Peak Testosterone readers are doing.
What is even more stunning is that a Low Carb Diet caused inflammation while losing weight! This goes against the conventional wisdom that any diet will improve health markers as long as you are experiencing weight loss. It takes a nasty diet indeed to increase inflammation under these circumstances. And that is exactly what a Low Fat Diet did in the above study: while CRP was actually lowered by 43% in the high carb, CRP in the Low Carb group increased by 25% even though the Low Carb participants lost more weight.  Ouch!
And here is a fact in a study that should catch every man's eye: endothelial vasoreactivity was reduced by the Low Carb Diet.  This means that the lining of the arteries were not as responsive to stimuli to relax and open and increase blood flow. That could easily translate to worsened erections. Again, high fat diets, from what I have seen on the Peak Testosterone Forum are almost always enjoyed only by the younger guys.
4. Too High in Carnitine? At least here in the U.S., most men consuming a lot of fat in their diet will also be consuming a lot of beef and pork. Sure, it's possible they could be slurping down coconut oil and/or fatty fish but this is unlikely from what I have seen. (Doing the latter could lead to low grade mercury poisoning as I discuss in my link on The Safety of Fish Consumption.) Most men are going to consume meat and, at least here in the U.S, two of the most affordable and well-liked options are beef and pork.
However, in 2013 researchers found beef and pork and unique in that they contain MUCH more carnitine than is found in any other dietary source. Furthermore, the carnitine uniquely fires up your gut bacteria which releases a chemical called TMAO that the researchers argued likely acclerates arteriosclerosis. For more information, see #3 in my list of The Risks of Meat Consumption for more information.
REBUTTAL: This is a controversial result and there are studies that show that various forms are carnitine are of cardiovascular benefit, so some experts are skeptical that it is actually the carnitine that is the underlying issue. More on that below.
5. Too High in Added Antibiotics, Hormones, . When the study in #5 came out showing that carnitine can cause arterial issues, the Paleo Diet gurus quickly rallied against it and attempted to counter the conclusions. And a good debate is always healthy of course. One of the Paleo objections was that modern livestock here in the U.S. are injected rather heavily with antibiotics and some of the antibiotics end up in the meat and can alter gut flora. In other words, the argument is simply that it's not the carnitine changing the gut but rather the antibiotics that is causing the problems.  However, this highlights a huge problem in my opinion with most men consuming Paleo and Low Carb Diets here in the U.S.: few men can afford to pay for range fed meats and end up eating factory farm livestock. These animals are corn fed. hormone and antibiotic injected, GMO BT toxins and arsenic (eggs and fowl). How is this going to be good for long term health.
1) http://well.blogs.nytimes.com/2012/11/07/can-exercise-protect-the-brain-from-fatty-foods, "Can Exercise Protect the Brain From Fatty Foods?"
2) J Am Coll Nutr, 2007 Apr;26(2):163-9, "Low carbohydrate, high fat diet increases C-reactive protein during weight loss"
3) Circulation, 2007, 116:II_819, "Abstract 3610: Comparative Effects of 3 Popular Diets on Lipids, Endothelial Function and Biomarkers of Atherothrombosis in the Absence of Weight Loss"