I have one, possibly two, unlikely three friends that know this about me.
Allow me to explain.
One night, while I still lived with 'the guys' (we all do this for a few years, right?) I was cruising an old, now disappeared Infantilism forum when a buddy came into my room. I left it up on the screen, since I figured there were no graphics to speak of, no avatars, or outlandish text. If he'd been paying attention, he could have read the word 'Infantilism', the only thing on the screen at the time which could have given me away. I sort of think he did read it, based on subtle remarks a while later. If he did figure me out, it doesn't matter to me. If I had to tell anyone, I'd have picked him anyway.
As for the second possibility (and this one is perhaps a stronger possibility): I never told my ex girlfriend, but I'd be pretty surprised if she hadn't connected the dots.
And the third...nah...I'll just keep it at two. Nobody, as far as I know, is aware of this part of me.