I have also read that circumcision results in a less sensitive penis during sex.
Duration of erection is unaffected, but the sensitivity of a circumcised penis is lower. The foreskin itself is sensitive, being fairly dense with nerve endings, and without one the most sensitive part is the glans. Now imagine if you were to walk around with your foreskin retracted, with the head of your penis rubbing up against your underwear. It might feel uncomfortable or even hurt, right? Many circumcised guys don't feel much of anything, or very little. During sex it's sensitive enough to achieve orgasm, but as a man gets older it gradually loses feeling due to dead tissue building up on the surface of the glans over the years.
A while after I started restoring my foreskin in my 20s, I was surprised that the sensitivity increased, so I do know firsthand that there's a difference.