Sigh. I'm sorry you are going through this. I was semi-lucky. My daughter snuck out just to be out, not to have sex but that was about the only difference. I finally got the house alarmed (her room, all the windows, the front, back and garage door) and would not give her the code. At least I knew when she was leaving and could call the police and report her as a runaway. They won't do much but they will record it, especially if the child is under 15.
If the boy is living at home, I would be banging on his parents' door every time my daughter sneaks out. Sooner or later they are going to get sick and tired of hearing from you and make his life miserable. If he's on his own, the best you can do is bang on his door every time she leaves without permission.
This is a truly awful idea but it might work. Have her taken in immediately after she comes home from one of her overnights for a vaginal exam at the ER. You would then have proof she is having sex and might even learn whether she is using any protection. This might get the legal system to do something to stop the boy (sorry, at 18, he's not a man in my mind).
I would definitely look into a CHINS/PINS petition. It might get you some help and at least put your daughter on notice that you're serious that her behavior cannot be tolerated.
Again, I'm sorry. I know how miserable you are. When my daughter was doing this, I ultimately had to find an Residential Treatment Center (RTC) for her. I do believe it saved her but it almost killed me to do that to her. HUGS