A different time, a different place. But this was in the decades before AIDS, when the "free love" movement of the 60s was still strong. Heck, my first time was during "the summer of love", and I hadn't had my 10th birthday yet.
More common, though, was the practice of " I'll show you mine if you show me yours" game children have played since time began. It's like we are born knowing where it's at, just not what it's for. Let us assume that scenario instead of my own.