One thing that has been mentioned but perhaps insufficiently emphasized is her actual age. How do you actually know that she is 18 (or whatever age where you are)? You should see her drivers license. If you were to have sex with her and she was not in fact 18, you would potentially be vulnerable to criminal prosecution. It would not matter if she had told you that she was 18.

You would also be vulnerable to her making false claims about your behavior. Even if you prevailed against her false claims, your reputation could be seriously damaged for having been so close to her.