Hi Moët. As you know, I have no magic answer
! Good idea to ask on this thread. People are so much kinder.
With DD, when the bf issue started I concentrated more on her coming home coz she had stuff to do - school work etc, and we liked to see her. Rather than saying I disliked her sleeping there.
Re: pregnancy. I'm afraid I took her to the docs and got the pill. She was unreliable with it so I took her back and she got the implant. And I gave her condoms. Didn't make a big deal - just handed them over. Did the same with DSs. I might be slated for that, and my God I never thought I would do it, but I knew they were having sex and I did not want an unwanted pregnancy.
However, she was 15 during all this so more compliant (though bolshier!). When she was 17 and repeated it all but more intensely with bf no2 I kind of let her go after a few months of rowing about it, and she came home much more after that.
You may find that her bf cools it a bit (bf no1 did) or that his parents get sick of her being round all the time and limit it. Or that it simply fizzles out, as these things often do.
It's very difficult to 'forbid' her at 17. You can of course withdraw privileges. DH wanted to go down that line, but DD would have just moved in with bf, and our relationship may have been ruined. I know also that it's really hard being surrounded by friends with compliant teens. But we're just not all that lucky! Maybe ours are more feisty and will end up changing the world, eh?!
Sorry you're going through this, I know how awful it is 