Is it illegal to see a teenage girl flashing?

Me and a friend were driving down the highway when a car of teenage girls pulled up next to us and one of them flashed us. We estimated her to be around 16 years old, and due to her age my friend said that by seeing this we had committed a crime. I disagreed, saying that it was no fault of our own, and we used no coercion or luring to get her to flash us. We're still debating the subject, and I was wondering if anyone had an actual legal answer for this?