RORY: Nothing like that [she and Dean being out all night] will ever happen again. I swear.
LORELAI: Don’t swear.
RORY: Why not?
LORELAI: Because you are your mother’s daughter.
RORY: What does that mean.
LORELAI: It means things can happen, even when you don’t really mean for them to happen.
Even though Lorelai believes Rory when she says nothing happened between her and Dean, she still doesn’t trust her daughter. That’s because she sees Rory as Lorelai Mark II, and as she doesn’t trust herself, she doesn’t trust Rory. Unfortunately she is setting Rory up to fail at relationships and birth control with her “no matter how hard you try, you will still screw things up” message.