Oh Chops! I like the idea of inviting her and another girl round though. Safety in numbers? If it helps (probably not a lot if I can sumarise as short as possible), when my sister was 14, very nearly 15, my parents were approached by a bloke (22 years old) and he asked if he could take her on a date. He'd seen her from my sister and I working in my parents shop and also when she had been walking home from school. I've never worked out why except my sister was particularly challenging at that age, but my parents said yes! I guess they too thought if they said no she would just go ahead and do it anyway. They started dating. A year later they got engaged and when my sister finished her gcse's she formally moved in with him (she was 16). When she just turned 17 they had a little girl. A pregnancy which despite my mother asking repeatedly, she denied until she was 6 months gone! Over the years they have had another daughter, got married and then another daughter arrived. Before my sister turned 30 she filed for divorce. I feel terrible as I once said I couldn't see them being still together when she was 30. Anyway, my parents found they weren't over fond of the guy as the years went on. But they couldn't tell her to leave him etc as of course they had allowed the relationship to start. All they could do was support my sister and the girls as best they could every time something went wrong, he lost his job(s), had an affair.
I guess what I'm saying is children may not make the choices we would like - my sister didn't get past her first term at college, but all we can do sometimes is be there to pick up the pieces and do our best to encourage them to see their own potential, not allow them to be sidetracked at important stages of their lives.
I know it's slightly different in that your daughter is embarking on a same sex relationship, but the success story is I have 3 gorgeous nieces and despite my sister dropping out of college, when her eldest was a year old she got a job, bought a house and has climbed the career ladder at work to management level with very good pay - she has long been able to support herself and the girls. It might be small comfort, but your daughter may just be pushing her teenage boundaries, especially as she knows it's a relationship which has caused problems in the past.