I thought it was a 3 year difference in most states?
Like 15+18, 16+19, and 17+20?
Just looked it up, there's another clause for over 21+under 16, but basically the younger one would have to file a complaint.
The only case I remember in recent years that got some media coverage was a teacher having an affair with a fourteen year old. She later went to the police, but for some reason I don't remember he got off despite the aforementioned rule.