The best thing to do is to talk to her, sit her down and explain to her that many young men at this time in her life will only want to have sex, because they are young and are not thinking of the responsibility. Of course she doesn't want to get a disease or have a baby, but those are the physical aspects of it, it's better to wait until she is older so that she can deal with the emotional aspects of it as well. Talking to her cannot be a one time thing, it has to be something that is done often. Once a week, once every few weeks. Talk to her not just about sex, but about whats going on in school, what her friends are doing, who her friends are, what her desires are in life.
Keeping an eye on her computer use (she shouldn't have a computer in her bedroom), watching who she is allowed to hang out with or where she is allowed to go, making good use of her free time, keeping an eye on phone calls, etc can all help.
Here are some links on how to talk to your teen about sex
The most important thing is creating a relationship where your teen knows that they can talk to you without you getting angry, yelling or beng judgemental. We may not like how they feel but we have to respect that they do feel. Being able to talk to them is the best way to avoid them making wrong choices. Listen more than you talk, be honest about sex and what it entails, and encourage her to talk to you.
I welcome your thoughts, let me know if you want to talk more.