I don't know why, but they just pop into my mind. I feel extremely guilty sometimes after masturbating while thinking about a nude 6-year-old or something. I'm kind of a hypocrite – when I'm not masturbating I'm truly against kiddie porn, but when I masturbate I get those images in my mind. Is there anything I can do to deal with this? Or should I just let these images go through my head and shut up about them?
As I've said before, fantasies, no matter how bizarre, are fine as long as they're just that – fantasies. Problems occur only when the fantasies start to creep out of your fantasy life and into your real life. I respect the fact that you say you're "against kiddie porn"; to me, that indicates you have good, sturdy boundaries between right and wrong, and between fantasy life and real life. But the more strange or socially unacceptable your fantasies, the more vigilant you have to be about keeping those boundaries intact and making sure they remain as such: fantasies.
Still, for many people persistent, socially unacceptable fantasies cause them to be unhappy with themselves. If this describes you, consider seeking out professional counseling. After all, nobody should be unhappy about his or herself because of what they think about when they're giving themselves pleasure.