Well... am going to be quite frank with you..not trying to be rude...but someone needs to spell it out.
First off...YOU ARE THE MOTHER!!! Why are you letting her make the rules? I'm sorry...but in my opinion, she should have never been allowed to have any male in her room period. When you caught them with the door shut after you told them not do...did you throw him out? Good kids or not...sex will get the best of them...it is not their brains they are thinking with!
Set the rules and make consequences and stick to them. Don't try to be her friend...be her mother. That means you will piss her off, she will say terrible things, and try and manipulate you...but remember, she has NO CLUE what she is doing...she is still a child and as your child it is YOUR job to do what you can to keep her from making preventable mistakes (i.e getting pregnant under your roof).
If you knew she was at his house...why on earth did you not call his mother, or better yet drive over there and drag her out by her ear?!?!?! Again, you are the mother? What was her punishment for it?
You can not control what they do when they are not home, but you have to maintain control over your own house. If they can not follow the rules, then he can not be there. If she is to be allowed at his house, then I would discuss the issue with his mother and lay out what your expectations are with supervision. If she does not agree or is not willing to do it, then she shouldn't go over there either.
The first thing that needs to be done is to get on the same page with your husband and you two come up with a game plan. Then both of you sit her down and lay down the ground rules. If she acts like a brat about it or is ugly, then she is not mature enough to have boys over or go over to boys houses. She will be understandably uncomfortable, but she will get over it and so will you. You have to be the adult in this situation.