Luke tries to give some Jess some guidance in regard to girls, and how they should be treated if you care about them. His lecture is thrown off the rails almost at once when Jess bluntly says that he doesn’t care about Shane. In fact, he can’t even remember her last name! If we needed any confirmation that Jess is just using Shane for physical pleasure and to make Rory jealous, here it is.
Hopefully Luke would have gone on to say that girls still deserve to be treated with decency and respect even when you don’t care about them, but he seems to be too shocked to continue further.