RORY: I do however reserve the right to change my mind.
LORELAI: That’s your prerogative as long as you remain a woman.
Lorelai is referring to the saying that “It is a woman’s prerogative to change her mind”. The proverb may come from law: from the Middle Ages onward, if a man broke off an engagement, he could be sued for breach of promise since he had broken a contract. But a woman was allowed to back out of an engagement with no legal repercussions, although there might be significant social ones.
Even though today both men and women are legally allowed to change their minds when it comes to marriage, the old saying remains.
The Annotated Gilmore Girls 