Seriously...lighten up. I understand that you are ashamed, but you have to know not to take things too personally on there internet...and ESPECIALLY here at Hip Forums.
i say go to church and confess your sin then pull your dick out at the pastor and ask him for advice.
rofl dude... kids do that shit all the time i could tell some stories... maybe some day i will but honestly, it's retarded to be so hard on yourself for that. ever hear of "playing doctor" or whatever? ...12 isn't that old. like i said, kids do that stuff. unless you're not telling us something like... you shoved it in her face or something... even then, i woulnd't blame a 12 y/o for somethign like that. it's called sexual exploration and aparently you didn't even really get into doing it.
or just say you had a over reacting bladder and had to take a urgent piss. and its something than runs in the family = even the pope well fall for this one.
dude, don't worry about people. someone is always going to try to turn everything into a joke. he went a little overboard, but you gotta get use to it. people won't stop just cuz you don't like it. however, i know how you feel about the guilt building up and wishing with everything you could go back and change what happened. but you can't. just forget about it. or deal with it. except it. whatever you want. it's really not that huge of an issue, but years of guilt have turned it into a big issue for you.
I really don't think you can be put on the sex offenders list now for something you did when you were 12. I sincerely hope that some day you may be able to overcome the incredible guilt. Have you tried apologizing to the girl? That might help immensely, get it in the air and off your chest.