Perhaps no loss of functionality, but there’s wide range of loss of sensation/enjoyment. It ranges from unnoticeably slight to almost total loss. Sources: Me, several gay friends, the men they’ve hooked up with, Internet strangers.

Then there was that guy who wrote in to Savage Love. His circumcision scar tissue was so tight that it caused his glans to become ischemic, necrotic, and then to fall off when he was an infant.