What happens when you go to your teenage daughters room and you find the door closed and she is in there with a boy?

Since when is that okay?  Strike One.  She is defying the house rules where there aren't boys allowed in her room.  I try to rationalize my reaction and figure out how to proceed.

Number one: the boy she is with isn't a boyfriend, at least not officially.  He is a great friend and one that I have always liked.  I think he respects my daughter, at least in theory.

Number two:  I was a teenager once and had the same rule but we had a basement so the bedroom was never an issue.  I was farther down the maturity road at her age, so I'm thinking I should cut her a break.  After all, she was in there when the entire family was home, how much can they really get into??
 
Number three: She is a great kid, with nice friends and good grades. She hasn't had a long-term boyfriend and could probably use the experience of cuddling with someone; it may even lighten her up a bit and get her out of her crabby mood.  Still, I am pissed that she broke the rules.

Still, I am pissed that she broke the rules.  Okay, What to do?  Knock on the door and time how long it takes her to answer?  Check to see how potentially disheveled she might look? Observe if the lights are still on?   Should I put my ear to the door first and listen first for any signs of inappropriate behavior?  EWW!  Don't really want to do that, but what the hell?  Before you can say surveillance camera my husband decides to go for it.  I run back to my room so it won't be overkill and totally embarrassing for her even though I am curious and annoyed.  The suspense is killing me.

My husband knocked on the door.  It was LOCKED. Strike Two. He looked at his watch to see how long it would take for her to answer.   She came to the door right away.   Clothes were on. No rustling overheard.  However the lights were very dim. Strike Three.  They were watching a movie.  She claims she locked the door to keep her pesky brother out of the room.  I ain't buying it.

My husband makes up some lame excuse for interrupting her, made a quick face at her after discovering the door was locked and they then came out of the room and hung out in a family room instead.  After her friend left we had a conversation about the rules and left it at a warning.  We cracked up as we noticed our parental hormones raging and once again became exactly what our parents were.  I only have 2 more years to watch our daughter navigate her teenage years...I just hope we all survive!