Gotta say, years ago when I was naked and ready to hop in the shower the doorbell rang. I asked who it was, he replied my paperboy. I knew him, a  nice kid about 12  I guess, and I said I was all set to hop in the shower, could be come back later?  He said it was OK, because he seen guys all the time in the shower at school, and it was OK with him if I was naked even.

At that I didn't mind, and as that door was at the bottom of a stairway I hustled up to my floor to get my wallet. I thought he'd wait at the bottom of the stairs, but he followed me right up. I went into my den to get my wallet and there he was standing at the top of my stairs, with an "Oh wow 'Mr. X'! "
SHIT!!!! I had a semi-boner! I I didn't know what to say, but said something like "Gee, I'm sorry Mike, but guys get like this sometimes, and you said you see guys in the shower at school, and probably your dad?"

At that, he just said he'd never seen an adult all naked, just his age, and never saw his dad naked. He said he never saw one as big as mine or with hair around it. By then, I had grown a full boner, sticking right up almost against my belly. By then I wasn't sure what to do, but decided to let him satisfy his curiosity, so just stood there letting him look. I told him I understood his curiosity, it was OK, I was curious like him when I was his age. At that I got my wallet and paid him, and told him it would be OK if when he collected next week he wanted to see me naked again. The sheepish grin on his face said it all and the following Tuesday I let him see me 'ready for the shower' again. 

As some weeks went on I let him feel my boner, and eventually got to stroke his too, just his pants pulled down, and by the time he was 13 he was ejaculating as we'd stroke each other there at the top my stairs and later in my bedroom.

Sure would never do anything like that these days!