I'll agree with everyone and also chip in to the better health theory. Obesity and being overweight do indeed cause problems with blood flow, and that is what an erection thrives on. The reason being, cholesterol cakes in the blood vessels, making it so that there is less space for blood to flow through, thus increasing the time it takes for blood to travel through the vessels and reach it's destination. Getting rid of cholesterol will make this problem go away, and you'll be getting faster and stiffer erections. Also, another major player in creating a harder erection is arousal, if you are extremely aroused, you can crush boulders with your erection. That may be a bit of an exaggeration, but arousal really does help a lot. C*ck rings, on the other hand, aren't quite so good for you, as they keep the blood in your penis, and your blood needs to flow or else problems can occur, because your penis needs the oxygen that blood carries, and if the blood isn't moving, no new oxygen can get to the penis after the blood in it already has run out.
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Mistermanpower.com and it's supporters are all full of bull.