[private]
[ he smiles, can't help it. ] Well... You eat with your mouth, right? But you also kiss with it. And breasts can be used to feed babies -- but those are still sexual, too. [ in fact, from what he's read... but we're not going to get into fetishes too much ]
A lot of our body parts serve more than one purpose. And the ones that we use for survival -- our mouths, our penises or vaginas -- they're sensitive in a good way. It feels good to eat, and it feels good to -- well, use the toilet. I guess maybe we... evolved that way, or were made that way, whichever you believe, so that we'd be sure to always do those things.
So touching those parts of our bodies -- and especially having someone else touch them -- feels really good.
I think part of the reason why sex is as intimate and special as it is, though, is because there's also something a little bit embarrassing about touching that part of someone else. Those parts of us are "private", and sharing them with someone else involves a lot of trust or love.