MAX: I’m the one that started the kiss.
LORELAI: And I’m the one that knocked it up to NC-17.
In the US, a rating of NC-17 means that a film is not suitable for children, and nobody aged 17 or under will be admitted to a movie theatre. It replaced the X-rating on films in the 1990s.
Lorelai is saying that their kiss is something the children shouldn’t have been allowed to see.