Daughter Had her foot down his pant?

So last hours of darkness My 16 year old daughter have her boyfriend in the living room watching movies and I be in my room. I walk out to get a drink within the kitchen and my daughter had her appendage down his pants. So i queitly walk out and went backbone in my room. She is my oldest. Im 36 years aged and divorced. Was that responsible of me? What should I have done? Should I speak to her roughly it? Should I tell her father? Not sure what to do. I come up with you should talk to her going on for it and get it out surrounded by the open. By not mentioning her behavior, you are silently condoning it. If you haven't already, speech to her about sex and her choices and speak about her that that type of behavior is unacceptable to you and that you hold guidelines and boundaries for what is allowed in your house and that you yearning that she would respect them.
Im sure they have already have sex. If you dont want her to be doing things such as putting her hand down her boyfriends pant, dont give her as much freedom. Forbid her have a boyfriend. Otherwise its going to happen because your allowing it.