I completely agree with JerryM. If the child is underage then everything in that phone is the parents business. IMO the phone is a privilege earned and if there are suspicions then the parent should look in the phone and if the child is being disciplined and the phone is the privilege taken away then the parent should look through the phone. There is a reason for a legal age and that is because before that legal age children and teens are not yet ready or mature enough to make big decisions like that. Yes, sex will come up in a teens conversation and yes some teens are sexually active, but IMO under the legal age children are not ready to make that decision.
I definitely agree on sex talks and education, keeping the communication line open, and making your child feel comfortable talking to you. And that can be done even if your child knows you won't hesitate to go through their phone. My daughter knows that I have every right to ask her or to check up on her any way that I want to and she recently told me that she loves talking to me about things because I always understand. She knows that there are boundaries and when those boundaries are crossed there will be consequenses. And she respects that.