First of all, think back to when you were her age. What were you like? What kind of 'talks' did your mom have with you? Did your mom overreact or prejudge? How would you have handled you differently?
I don't mean any offense by that. I just think that we as parents need to reflect once in a while in regards to our kids. A lot of the same principals still are valid. No matter how much the younger generation insists "things are different now", it rarely turns out to be the case. Same behaviors, same reactions, same concerns.
So sit down with your daughter and explain your concerns. If you plan to set limitations or boundaries, explain that to her and why. If she breaks the rules, explain the consequences and don't deviate from them. If you didn't care, you wouldn't be having the concerns or conversation.
Explain the vulnerable time of life she's in and it's easy to get a broken heart. Although it does happen that great marriage start at ages this early, it's a very rare thing. And there are millions of people out there to meet and that makes it difficult to meet them if one ties one's self down to just one.
Young blossoming love and affection like this is real. It's a beginning. And it's a process in learning what traits a person likes in the opposite sex. And it may only take one person, or it may take a lot of boy/girlfriends to 'gather all the evidence' and decide what kind of a person one wants to ultimately settle down with.
Give her a great big hug. And see if there isn't a way to prevent your heart from breaking or the trust between you from breaking. If she's as smart as you say, a good head with good guidance will keep the worries away.