There are basically two reasons to eat a Low Fat (or Ornish) Diet: 1) your penis and 2) your penis. You can probably guess the third reason as well.
The reason I say this is that a typical High Fat Diet/Low Carb Diet has negative short term AND long term effects on your blood flow. First, let's talk about the short term reasons. Researchers compared for six weeks a Low Carb (or High Fat) Diet versus a Low Fat (or Ornish) Diet. The study clearly showed less Nitric Oxide and decreased blood flow for the Low Carb (or High Fat) Diet.  The lead researcher of this study warned that "the reduced production from the endothelium of nitric oxide, a specific chemical, puts the vessel at higher risk of abnormal thickening, greater clotting potential, and cholesterol deposition, all part of the atherosclerosis process". That's a scienfically polite way of saying that a Atkins Diet will clog your arteries, slow blood flow and make your arteries less elastic, i.e. temporarily hardened. This, of course, is the antithesis of what you need for a good sex life. (Read my Why Satured Fat Can Be Bad for Men link for even more details.)
Now this is very bad: you want lots of Nitric Oxide and lots of blood flow between your legs if you know what I mean! The typical Low Carb (or High Fat) Diet will not do this for you. He also warned that the Low Carb (or High Fat) Diet had much less folic acid, which could easily lead to increased homocyteine levels. (Homocysteine is a leading risk factor for heart disease.)
Besides these short term blood flow and nitric oxide issues are long term issues that will definitely effect your sex life. First of all, cholesterol levels are directly and almost linearly associated with Erectile Dysfunction. And guess what study after study has shown raises cholesterol? You got it - saturated fat. And saturated fat is normally the fat associated with most High Fat Diets.
And here is my point: cholesterol levels are directly and almost linearly associated with Erectile Dysfunction. For example, researchers found that for about every 35-40 points of increase in total cholesterol, there was a 0.38 times increase in erectile dysfunction risk!  That makes sense, because many studies have reavealed that men with cholesterol at or below 150 have virtually no heart disease. That this should translate to better erectile function was shown in a study where researchers studied a group of men with erectile dysfunction whose only risk factor was high cholesterol. These men were given a statin drug and erectile function improved significantly.  Furthermore, one study showed that exercise coupled with a low fat diet led to great improved blood flow and that that will do nothing but improve your erections. 
What this means is that long term, the Low Fat Diet is the clear winner over the High Fat/Low Carb Diet as far as erectile strength and cardiovascular disease. Probably the most famous of all the Low Fat Diets is the Ornish Diet. Dr. Dean Ornish popularized a low fat, mostly vegan diet (coupled with stress management) that significantly reduced total cholesterol, LDL and blood pressure and, most notably, actually decreased arteriosclerosis . The participants were allowed some animal products, egg whites and low fat dairy, but for the most part they ate beans, legumes, whole wheat grains on top of vegetables and fruit. Absolutely forbidden was anything with significant amounts of fat, including fish and nuts.
Many of you will find drastic improvements in your erectile dysfunction if you'll just adopt a Low Fat Diet. For more information, read my links on the The Many Benefits of a Low Fat Diet and Great Diet Smackdown (Part II). Some of you will get almost immediate relief by simply breaking the High Protein and High Saturated Fat habit.
High Protein and High Saturated Fat usually go hand in hand due to meat and dairy consumption. High Protein diets have now been implicated as well, decreasing blood flow in a recent study.Furthermore, the authors pointed out that "fibrinogen, Lp (a), and C-RP increased by an average of 14%, 106%, and 61% respectively".  This is a one way ticket to heart and penis problems - avoid it like the plague. Also, remember that saturated fat raises your cholesterol and literally hardens your arteries temporarily. I know that high saturated fat diets are the rage right now, but for most guys, this is nuthin' but trouble for their sex life. We know many guys who after a week on a plant-based diet cured their erection problems. If you just gotta have fat, use olive oil or have a few almonds and walnuts.
NOTE: Don't believe the bad press that is floating around the web: read about How Incredibly Healthy Grains Are for you.