I think she realized her mistake immediately, because a few moments later she awkwardly and self-consciously added something like "...yeah not until least the 3rd date."
The problem with telling a guy when you'll have sex with him is that you give him expectations. If I'd gone on a third date with that girl, you better believe that I'd have been expecting sex. Even if she didn't feel ready, she would feel pressured by her own words to sleep with me. Of course she would always have the right to say no, but it would cause friction in the relationship ("but you said...").
1. Don't Fuck on The First Date
2. Don't Initiate Contact
3. Never Tell a Man Why He Shouldn't Want to Date You
4. How to Turn Down a Guy for Sex