No, seriously, as any psychology study will tell you, it's pretty much a universal that brothers and sisters of about the same age will, at the very least, have a "wtf is this thing?/don't touch it!" moment while "playing doctor" together, or in a bathtub, or some shit, lest their parents watch them like hawks. It's part of growing up, and if no sibling is about, it'll likely be a friend.
Now, it almost never leads to actual sex - if it does, it generally means there's been some sort of abuse elsewhere in the family, and it's merely continuing a chain via learned traumatic behavior.
If that happened, I suspect maybe abuse runs throughout Adam's family. He does seem a bit timid, but I've never gotten that vibe from him - though it's been a long time since I practiced psychology, so maybe my radar is rusty, and I've only seen maybe two or three episodes of his show.