The pressure of getting a long-lasting erection or having a big enough penis can become such a focus to a man (and his partner) that it dominates his experience, not allowing him (or his lover) to be present, connected, and all of the other things necessary for an amazing sexual experience.
Erectile dysfunction can be a sign of underlying health problems and sufferers of ED could reap cardiovascular health benefits
Throughout my youth, I was both fascinated and jealous of the relationship men had with their penises. As a result you could have knocked me over with a feather when I found out men are just as uptight about their penises as women are about their vulvas. It's just a different kind of uptight.