Actually...
When I was pre-pubescent/adolescent this was a bit of a fantasy!
I remember my friend told me about a movie, I believe it was called "ice pirates", about women ruling the world and cutting off men's dicks, and I was strangely aroused by the thought.
And
I have read a story where a gay couple did just that. Except the one guy flushed his penis down the toilet. There are also quite a few gut churning mutilation pictures and videos on the internet, as well as testimonials of voluntary erotic castrations.