We can't literally prevent our kids from having sex, doing drugs, or acting irresponsibly in any number of ways. We can, however, communicate our beliefs and values regarding sex in a way that is caring and not condemning or judgmental. Kids at this age are faced with attempting to create an identity that will be accepted by their peers; their sexual identity is a big part of that process.
Don't just give your daughter rules and limits about sex, tell her what you believe. Discussing how you feel sex can be part of a healthy relationship. Let her have her say and don't condemn her for having different ideas on this subject. Shaming her or grounding her will only shut off your connection. Lots of teens are woefully ignorant about sex, even though they want you to believe they know it all. You need to communicate sexual facts to your daughter so she has the knowledge to prevent disease and pregnancy.
These are the toughest conversations for parents to have with their kids but they must take place. Remember, your daughter may currently be making unwise decisions regarding sex, but that doesn't make her a bad kid. She needs you more than ever now, even though she will tell you to leave her alone about this. Don't be afraid to enlist the help of any family or friends whom she respects to help you with this situation.