It's a mental thing. You have to treat what she said like a "shit test" and ignore it. Act as if you never heard her say it, and proceed as if sex with her on the first date is still very possible. If she asks you something like "Did you hear what i said? I told you that i don't ever sleep with any guy on the first date!" Don't get reactive to it in any way, just calmly acknowledge it with a "Ok, i understand" statement, just to appease her ASD (Anti-Slut Defense) so that she can let her defenses back down again. But assume that it's still on with her anyway!..regardless of what she says.
Always maintain your frame, never fall into her frame.
-G