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There are people that were circumcized after becoming sexually active who can (and do) report exactly that.
Separately, we can simply ask people that have foreskins to describe the sensations they feel from that body part.
The only part that we can’t say with confidence is how the neutral pathways develop (i.e. how we perceive the sensations) when it’s the only way we’ve ever experienced.
It’s also at an age when your libido is essentially unlimited. Were the opportunity to present itself, these people could adequately perform for 15 different partners in a row.
Desensitization takes time. By the time it sets in, most people are past the age of unlimited libido. There are also different forms of sensitivity (and thus, desensitization). OP referred to physical, but mental seems to be a more common issue.