I'm not in the same situation but just wanted to extend a hand of friendship so you don't feel quite so alone.
I had something similar when one of my daughters went out with a bloke who we could all see was an absolute waster but it took her ages to see it.
I didn't ban him from the house or seeing her but I was totally honest with her and when he (or she, for that matter) did something I disapproved of, I told them. I wanted her to be able to compare the difference between our set of values and his and make a choice.
I'm pleased your daughter has enough maturity to put herself on the pill (see, you brought her up sensibly if she thought of that) but make sure she realises about the chance of STD's too.
Does she listen to you at all? If so, you could try sitting her down, ask her about her plans for the future and listen to her. Try to point out the problems to her gently so you plant a seed in her mind about just how hard her life is likely to be if she persists in leaving home at such an early age. If you're on good terms with her friends, perhaps you could ask them to talk to her as she's more likely to value their opinions.
I hope some of these suggestions may help. I understand your feelings but now you have to be strong and calm for her. Make sure she realises there's always a safe place for her with you.
I wish you all the best.