caught my daughter having sex

My 17 year old daughter was home alone, I trusted to stay by herself, when I walked in a boy was leaving. I confronted my daughter about it and she told me the truth but I feel so betrayed this isn't the first time she's done it either. 
She's on birth control, but what should I do?? Should I punish her again?? I did that and it obviously didn't work. Help?