Here's a story I found in JUGHEAD WITH ARCHIE #75, dated July 1986. Since it's a reprint digest, I don't know when it was first published. But I'm surprised that (1) it was published at all, and (2) it was reprinted, and (3) it made it past the Comics Code Authority.
Now granted, Archie and Betty are talking basically in "code" here, but they're still talking about having sex. Given that most "first time" stories are about whether or not they're actually ready to do it, it's hard to read Archie and Betty stories now without thinking they get naughty off-camera quite a bit. They're clearly ready here.