The act could have happened months before hand, just after the guy turned 19 and the girl was still 16, or in a jurisdiction that ignores the Romeo and Juliet clause. Or the entire thing could have happened years before when he was 17 and she was 15, with the parents only finding out about it after she turned 16 and the investigation another year later turned up evidence to support that theory.

Also, the GP to this thread never stated that the guy would be a convicted statutory rapist for sleeping with his wife, but that he would have been a statutory rapist who's victim is the person who is now his wife.

There's plenty of ways this could have worked to the guy's disadvantage. There's really only one way it can be false.