I've found the best time to tell someone about a fetish is after the 3rd bang. You like each other well enough to have sex, but still have an opportunity to air things out before you're "set" in the relationship. The trust issue is important, so for sure don't say "i want to cut off your dick" to anyone you just met. Unless you met them on this forum.![]()
Since true penectomy (for anything other than medical reasons) will probably land you in prison, stick with knifeplay. It's exciting as hell and you can come back and play more. Try a blade in bed rubbing it on his skin... start at the chest, go down, and refrain from actual bloodletting. See how that plays out. If he's into it, you'll know.
http://www.angelfire.com/al4/knife_lessons/intro.html