Why are Viagra and Cialis such powerful cures for erectile dysfunction? The answer is simple: they affect the arteries. The problem is that Viagra and Cialis often do not work. There can be a number of reasons for this, but the most common is that some men simply do not produce enough basline nitric oxide (NO). Viagra and Cialis work on preserving cGMP, a molecule triggered by NO. I cover this in my page on Viagra Failure for example.
Now stop and ask yourself an all-important question: why would a man not have enough baseline nitric oxide? The answer is usually atherosclerosis, the buildup of arterial plaque. When the lining of your arteries, the endothelium, is covered with plaque, how is nitric oxide supposed to be released in youthful quantities? It doesn't! Atherosclerosis in your penile arteries is going to decrease nitric oxide levels and slowly decrease your hardness factor. In addition, the plaque buildup can make arteries less flexible, which is so important for improved blood flow. The bottom line is that, for most men at least, the best, most efficient way to improve bedroom performance is to focus on reducing arterial plaque and other natural techniques for improving blood flow and nitric oxide. Of course, the big problem is that the Western lifestyle is incredibly hard on your arteries. In fact, it's really hard to imagine designing a way of living that would be worse. (Yes, hormones are VERY important to erections. But, in my opinion, nitric oxide and arterial health are kings.)
NOTE: This is a vicious circle, because nitric oxide helps protect against arterial plaque through it's anti-inflammatory properties, i.e. plaque lowers nitric oxide and lower nitric oxide increases plaque.
DO I HAVE PROOF? I know that many men are skeptical that arterial plaque could be a root cause of troubles in the bedroom, so let me offer Four Reasons to Believe Arterial Plaque Usually Causes Erectile Dysfunction::
1. The Pudendal Artery Study. Ever heard of the pudendal artery? Neither had I. It turns out it is the most important artery in the body, because it is the one that supplies blood to the penis in the pelvic region. Researchers recently performaned angiograms (to estimate arterial plaque)  of the heart and pudendal arteries of men who were not responding to PDE5 inhibitors such as Viagra and Cialis. What they found was direct evidence that atherosclerosis was responsible for their severe erectile dysfunction:
"Rogers and colleagues have already completed a pilot study, called the Pelvic Angiography in Non-Responders to PDE-5 Inhibitors (PANPI), which correlated angiographic evidence of coronary disease with pudendal arterial disease. In PANPI, 10 patients undergoing coronary angiography for CAD symptoms who also reported a poor response to PDE-5 inhibitors underwent a pelvic angiogram as well. Results showed that stenosis [narrowing] in the coronary arteries typically mirrored that of the pudendal artery, which ranged from a mean of 52% in the right internal pudendal artery to 60% in the left. We basically found a 100% correlation" between coronary artery disease and pudendal artery disease..." 
Just to make sure this is clear, they found that plaque in the cardiac arteries, which is what can kill you from a heart attack, was strongly associated with plaque in the artery that supplies blood to the penis, which can kill your sex life. The authors gave some other profound statistics:
--4 out of 5 cases of erectile dysfuncton have to do with the arteries.
--70% of men with heart disease (significant arterial plaque) also have erectile dysfunction.
Often men are focused on hormones. Why? Because that is easier in many ways. If your testosterone is low, you can simply take testosterone, Clomid or HCG for example. However, it is non-trivial to reverse arterial plaque. It usually requires weight loss, careful attention to diet, and possibly a couple of supplements, something I cover in my pages on Penile Arterial Plaque Regression. It also takes time - months and months or even a few years depending..
2. The Connection of IMT to Erectile Dysfunction. Younger men in particular can boost their testosterone or perhaps lower their estradiol and get back in the game. But, if plaque is building up in their arteries, they are simply delaying the inevitable. Look at what this man wrote on The Peak Testosterone Forum:
"I agree peak my problem probably is not testostorone. But I have been extremely active and fit over the last 4 years and have been trying even harder. Could i still have atherosclerosis? I have lost weight over the last 6 months from 102 kg to 85kg and in the best of shape. I know sometimes even fit athletes have atherosclerosis too." 
The answer to his question is 'yes', he could very likely have atherosclerosi,s especially since he has E.D. Studies show that about half of young men in their 20's have significant plaque. I have a number of pages on the subject and those interested may want to read my page on How to Clear Your Arteries.
One study showed the erection-to-arteriosclerosis connection particularly well. It looked specificially at the association by examining men with erectile dysfunction but not clinical atherosclerosis (which probably means clinical loss of blood flow to the extremities or the heart).  Basically, they were looking at men in the earlier stages of erectile dysfunction.
What they found was that when their IMT (intima-media thickness) was examined - IMT is a measure of the amount of plaque in the neck artery - there was a significant correlation to the amount of erectile issues the men were experiencing. Typically, IMT measurements are done by ultrasound and on the large arteries that go up your neck and they are a reasonably good gauge of the plaque buildup in your extremities, such as the arms, legs and penis.
So what exactly did the reserchers find? First of all, they actually evaluated both VRF (vascular risk factors) and IMT. The correlation was interesting:
"Men with VRFs (vascular risk factors) and a high carotid IMT score demonstrated more severe ED, were older and had a higher serum level of C-reactive protein compared to men with VRFs and an IMT of less than 1.00 mm."
"A high IMT score but not an increased measure for each VRF, including aging, significantly increased the risk of severe ED (odds ratio 2.6, confidence interval 1.1 to 5.9) even after controlling for smoking and drugs associated with ED."
In other words, men with significant cardiovascular risk factors and arteriosclerosis had the worst erectile dysfunction. Is this any surprise? Basically, the link between arterial health and E.D. was verified by the fact that 1) as atherosclerosis increased, erectile dysfunction rose and 2) as cardiovascular risk factors increased, erecitle dysfunction increased (in many circumstances).
NOTE: They also found that high inflammation, i.e. a high CRP score, was associated with most severe erectile dysfunction (as well as general aging). Inflammation plays an important role in the buildup of arterial plaque and you can help protect yourself by scanning through my Lowering Inflammation 101 pages.
3. Risk Factors for Heart Disease are Risk Factors for Erectile Dysfunction. If heart disease really is the general root cause of erectile dysfunction, then one would expect that the risk factors for heart disease would also be risk factors for erectile dysfunction. This is indeed the case for almost all the major players. Here are just a few examples:
a) CRP (Inflammation). Researchers split men with no and severe erectile dysfunction into two study cohorts. What they found was that the men with severe E.D. has over four times the CRP levels of the men with no E.D. They also estimated the plaque in the penile arteries using ultrasound and found that "CRP levels correlate significantly with increasing severity of penile vascular disease as measured by penile Doppler." 
b) Prediabetes (Metabolic Syndrome). Men with prediabetes had double the risk of erectile dysfunction when compared with age and BMI-matched controls. 
c) High LDL and Cholesterol. I know that some men will point out correctly that LDL-P is a better predictor of heart disease than LDL-C, the "regular" LDL, and total cholesterol, something I discuss at length on my site. However, the fact is that most men with high "regular" LDL or cholesterol will also have high LDL-P and that is the point of this study tthat found that their "results showed that there was a significant difference between mean plasma cholesterol and LDL levels in the individuals suffering from ED and the control group." 
d) Smoking. Smoking, interestingly enough, slightly increases testosterone usually and so it should be good for erections, right? Wrong! Smoking is incredibly hard on the lining of the arteries and a big risk factor for heart disease. And, as expected, it is a also a risk factor for erectile dysfunction according to a number of studies. For example, the well-known Massachusetts Male Aging Study found that "Cigarette smoking was associated with a greater probability of complete impotence in men with heart disease and hypertension."  In other words, cigarrette smoking is great way to accelerate a complete loss of erections!
e) Apnea. Another example is apnea. While it is true that apnea lowers testosterone somewhat - see my link on Apnea and Testosterone - the usual drop is "only" about 30%. Most men can lose 30% of their testosterone and not suffer erectile dysfunction. However, apnea has an even more sinister side, because it causes the release of a peptide..
NOTE: Again, I am NOT saying that hormones are not important. Testosterone now has several solid studies behind it showing that, in most men anyway, TRT can improve cardiovascular disease and, of course, it can also improve erections. See my page on Testosterone and Heart Disease for a more complete discussion. (I personally believe that a small percentage of men may be at risk, something I discuss in my page on The Potential Risks of Testosterone Replacement Therapy. Also, there is a very interesting study out there that shows men with hypertension has little increased risk of erectile dysfunction when controlled for testosterone.  Keep in mind, though, that this is an older study and a lot of other research shows high blood pressure is associated with E.D.
4. Erectile Dysfunction Predicts Heart Attacks. Suppose just for a minute that erectile dysfunction is a sign of failing arterial health as the above suggests. If this was true, then we would expect that erectile dysfunction would actually be a valid cardiovascular risk factor. And researchers have found that this is exactly the case. In fact, erectile dysfunction is such a powerful predictor that one study found that men had, on average, about five years after developing E.D. before the occurrence of a cardiovascular event such as an M.I. (heart attack), etc.!
This means that, generally speaking, if you have developed erectile dysfunction, you have five years to fix the problem or it could be game over around five years later. Fortunately, this also gives you time to change course and take care of the underlying cardiovasclar issues. For the studies that support this, see my page on The Dangers of Erectile Dysfunction.
2) J Urol, 2005 Feb, 173(2):526-9, "Intima-media thickening of common carotid arteries is a risk factor for severe erectile dysfunction in men with vascular risk factors but no clinical evidence of atherosclerosis."
3) J Urol, 1994 Jan,151(1):54-61, "Impotence and its medical and psychosocial correlates: results of the Massachusetts Male Aging Study"
4) J Endovasc Ther, 2008 Feb, 15(1): 117 125, "Angiography Underestimates Peripheral Atherosclerosis: Lumenography Revisited"
5) http://www.medscape.com/viewarticle/709967 , Shelley Wood, Oct 05 2009, "Drug-Eluting Stents for Erectile Dysfunction"
6) International Journal of Impotence Research, 2005, 17:523 526, "The relationship between lipid profile and erectile dysfunction"
7) Diabetes Care May 2005 vol. 28 no. 5 1201-1203, "High Proportions of Erectile Dysfunction in Men With the Metabolic Syndrome"
8) International Journal of Impotence Research, 2003, 15:231 236, "Relation of C-reactive protein and other cardiovascular risk factors to penile vascular disease in men with erectile dysfunction"
9) Hypertension, 1996, 28: 859-862, "Erectile Dysfunction in Hypertensive Subjects"