How would you know the difference in sensation if you've never had foreskin though?
I have been circumcised as an adult and I can tell you that you DO lose some sensitivity on your (to use a medical term) 'bell-end'.
Best way to describe this is to stroke the palm of your hand with your finger and then stroke the back of your hand. You will notice the difference in sensation, which is analogous to pre and post-circumcision sensitivity.
Good news is that sex is still loads of fun.
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So don't get downhearted.