Can eating spinach help erections? Absolutely! And the way it performs this bedroom-enhancing phenomenon is primarily by being one of the higher-nitrate superfoods. Few foods have more nitrates than spinach - no one can beat arugula lettuce however! - and this is the reason that researchers recently studied spinach's ability to raise nitric oxide and lower blood pressure. They took 26 healthy men and women between the ages of 38 and 69 and gave them enough spinach to get 200 mg of food-based nitrates and then matched them against a non-nitrate diet. 
The results were impressive:
Now what is so impressive is that they achieved these results in healthy men and women. Many studies show that those with prehypertension and hypertension get much bigger drops in blood pressure from treatments both natural and pharmaceutical. And this is exactly what I have seen on the Peak Testosterone Forum, i.e. big improvements in men struggling with medical issues such as erectile dysfunction - which usually has an arterial / cardiovascular component - and higher blood pressure.
And, of course, it is also impressive that they achieved these results with just one dietary modification: adding some spinach to their meals. Plant foods have many other properties that increase blood flow and blood pressure, and this is the subject of my book The Peak Erectile Strength Diet. Again, many plant-based diets, such as the DASH Diet and a Low Fat Diet, have been shown to yield nice drops in blood pressure. In fact, one study showed that even flavonol-rich apples resulted in a nice increase in salivary nitrites.  Spinach outperformed the apple in this study, but the primary point is that a diet rich in plants will maximize all the parameters that you need for erectile strength. I can only say to just try it if you're not eating a lot of vegetables and fruits (and brown rice) and you'll likely immediately notice a difference.
Spinach also may provide some long term protection against arteriosclerosis according to one animal study.  It also likely protects against clots and evidence also shows that it reduces the risk of stroke.  In other words, spinach looks very good as a general Artery and Cardiovascular Healer.
I can also tell you anecdotally that eating high nitrate foods has dropped my blood pressure by 10+ points. In my page on How I Boosted My Nitric Oxide, I discuss an arugula-eating experiment that I did. Beets will also work and beetroot juice in particular has been in the health news lately. Beets are also high nitrate foods and the juice is particularly easy to consume. For this reason, researchers have studied beets in detail and the results have been quite impressive, something I document in my link The Benefits of Beetroot Juice.
Just one example: researchers recently did a study of beetroot juice on participants with high blood pressure using the same amount of food nitrates (200 mg) as the above spinach study and the results were also impressive:
These are impressive results from drinking a mere 8 oz. of beetroot juice and were documented in the presitigious journal Hypertension. Again, the results achieved are not too far off those from standard medications. For another great blood pressure-lowering nutraceutical solution, see my page on Garlic and Vitamin C.
1) Nitric Oxide, 2013 Nov 30, 35:123-30, "Effects of a nitrate-rich meal on arterial stiffness and blood pressure in healthy volunteers"
3) Free Radic Biol Med, 2012 Jan 1, 52(1):95-102, "Flavonoid-rich apples and nitrate-rich spinach augment nitric oxide status and improve endothelial function in healthy men and women: a randomized controlled trial"
4) Atherosclerosis, 1972 Jan-Feb, 15(1):87-92, "Effect of spinach and wakame on cholesterol turnover in the rat"
5) Journal of Medicinal Food, Jul 15 2011, 14(7-8), "Dietary Spinach Saponin-Enriched Lipophilic Fraction Inhibits Platelet Aggregation and Blood Coagulation"
6) http://usatoday30.usatoday.com/news/health/diet/2002-05-03-spinach-stroke.htm , USA TODAY05/02/2002, "Spinach may reduce stroke risk", By Kathleen Fackelmann