This is very much a fetish of mine, although my focus is mainly on the foreskin (and its possible removal) rather than on cut cocks, which I find much less interesting and attractive than intact ones.
I remember as a young boy being fascinated by the variety in appearance of the foreskins of my schoolmates, and by the fact that a few boys (and my father) had a different type of penis with a permanently bare knob at the end. With a few of my closer friends I was also able to discover the variety in shape and colour of the knob which could be uncovered by most boys, and I found that some boys were unable to pull the skin back, or could only reveal part of their knob. At that age I think it was just an extreme curiosity, rather than anything sexual, although I got erections when playing with my own penis.
The next (and undoubtedly the most important) step in the development of my fetish was at the age of 11 or 12 when my previously loose foreskin became progressively tighter until it was completely unretractable. At first I thought this might only be temporary, and as I knew that a few other boys were unable to retract it didn't worry me too much.
However, later in my teens it became clear that my foreskin was not going to loosen of its own accord. I had learned that the cure for this condition was circumcision, and the prospect of this terrified me, so I kept it to myself. I consulted medical books in libraries (this was before the internet) and found myself being aroused by the technical terms such as phimosis, paraphimosis, glans, frenulum, corona, sulcus etc. Their advice was always the same: if you can't pull back your foreskin you need to be circumcised.
My teen masturbation fantasies began to involve carrying out medical exams on all the other boys in my class, the highlight of course being a check of the foreskin for retractability and cleanliness. If necessary, force would be used to the point of pain, and boys would be sent to be circumcised if I considered the skin to be too tight or too dirty/smelly. I don't think I ever fantasised about carrying out the operation myself.
I became increasingly frustrated by my tight foreskin and eventually got the courage to undergo circumcision at the age of 31, nearly 32 years ago. My fetish has remained strong ever since.