There's some good thoughts and insight on this thread, freaklikeme.
My DD17 has been with her first ever proper BF18 for about 5 months.
I encouraged her to invite him to our house right from the start and I've always tried to make it quite attractive for them to spend time here. We give them plenty of space, I make a nice family supper, let them have a glass of wine or a beer... They do go out - both as a couple and as part of a crowd of friends - but I'm happy that they also like spending time hanging out at our house (they hang out at his too, but mostly at ours) because I like having them around and under our roof. And, frankly, I like seeing him too - he's lovely. I love all of her friends actually - they're all delightful.
I would say that my DD and I are quite close so I have delicately fished around all along, asking questions about how things are going. I've tried to be very open and positive and light all along about the whole thing and I think it's encouraged her to talk. I've told her it's a learning curve for her but also a learning curve for me and that I'm pleased and excited for her - that a wonderful romance with a gorgeous boy is one of life's great pleasures.
As concerns sex: My opinion is that teenagers do what they are going to do whether you know/approve about it or not. Over the years she has told me about umpteen girls who are having sex (in many cases, this was underage) and their Mums didn't/ don't have a clue. I've always told her that while I'll have my own views about what would be the right (and wrong) situations, I'd rather she stay close to me about the reality of her life so that I can support her and help keep her safe rather than to have to sneak around behind my back. Mostly, I've made it very clear to her that she can trust me not to flip out.
She told me when they started to experiment and I have (I hope) been supportive and non-judgmental. She talked with her BF on many occasions about sex and he assured her that there was no pressure from him, he would wait a year or however long she needed to feel completely ready but that when she was ready, he would always have condoms. They are both intelligent and responsible kids and she has assured me that she would absolutely not have sex without a condom. Despite his reassurance that he would always have condoms, I have, nevertheless, given her a couple to keep up her sleeve as a safety net in case there is ever a time when he's run out and they are tempted to think that it's a safe time of the month or whatever.
It's a tough line to tread... I told her I would prefer her to wait until they had been an item for six months but, at seventeen, it's her body/ relationship/ decision and, a few weeks ago, she lost her virginity. I sort of sensed that it was imminent and could tell as soon as I saw her face.
She knows that I would have preferred her to have waited for a couple more months but she is completely happy with her decision, so there we are. There are worse situations - she's seventeen and in a lovely, steady relationship with a great boy who treats her very well... I think it's very, very normal.
My advice to you, freaklikeme is to love and support your daughter and encourage her to stay close Make him welcome and gently encourage them to spend time in your home. These things rarely last forever so hopefully you can all enjoy it while it lasts.