I told my DD that if she was having sex because she really wanted to and felt ready then I had no problem with it. I didn't want her to have sex just to feel grown up, as an act of rebellion or because she felt she ought to for any reason.
A bit of me felt sad at the thought of her being involved in all the rubbish stuff that goes with having sex - having to sort contraception, the possibility of health problems such as potential problems with the pill, cystitis, possibility of pregnancy obviously and fearful for how it would be when the inevitable break-up came, but in all honesty I also felt happy for her as she was clearly in love with her lovely bf and it's lovely to be young and energetic and not have any major responsibilities cramping your style.
I was genuinely pleased that she was enjoying her teenage years.
Like a pp, I told her that porn and sex in films is very unrealistic and she should never ever feel the need to do anything she didn't enjoy.
I also told her about a website called Scarleteen for young people, which has very detailed, sensible info about all aspects of relationships, as I'm not an expert. 
I didn't stop her staying over at her bf's home and he stayed over at our home. I could have done without him appearing in the house on a sunday morning sometimes, but as she was 17 and heading for uni soon I chose to put up with it as I knew we wouldn't have too long of it. It was much handier than doing lifts late at night - not walking distance. If he lived closer I'd have said there was no need - go home at the end of the evening.
Set rules. Once DD's BF spent the whole weekend here and I was really longing to get the house back to ourselves. They didn't stay over on a school night.