P.M.
Um, I'm still wondering this about DH ;)
Really though, this is still normal. My son is almost 5 and several times a day I lean down and gently whisper to him, "Hey buddy, that's for private time." He always takes his hand away and laughs, "Oh yeah, I forgot!"
It's a habit thing. They don't really think about it when they do it. No biggie, you and allllll the other mommas of little boys all deal with this one ;) Good for you for not freaking out.