I can by no means answer this question scientifically. There are probably many reasons why circumcised men defend the practice of male genital mutilation, and it would require an extensive survey to determine the answer. However, even such a survey would be suspect, particularly if the following hypothesis is correct. I base the following hypothesis on my own experience.
I think there is, among most circumcised men, a (possibly subconscious) desire to believe circumcision is harmless. This is because we do not like the idea that there is an aspect of sexual pleasure from which we have quite literally been cut off, not to mention the idea that circumcision is detrimental to the pleasure of our partners. So when introduced to the idea that circumcision is bad for both the victim’s and his partners’ sexual pleasure, a circumcision victim’s natural reaction is to want to deny this, not out of any rational consideration, but because he so desperately wants it to be untrue. This was certainly my own first reaction, as I now realize.
This drive leads to outright denial of facts and evidence, as well as to confirmation bias, and all the other irrational aspects of analysis to which the human mind is subject. Trying to argue with anyone who has a need to believe something is very difficult; this need to believe will trump evidence, and they often do not even realize that it is this need which is driving them to reject the evidence. Therefore, if surveyed, they would likely not report on this true cause, or even be aware of it.
(On a side note, this is a good reason to disregard personal narratives, which are considered so important in postmodernist ‘studies:’ the fact is, we often misunderstand ourselves.)