Doctors, nurses, school secretaries - even IRS agents - don't get to decide which couples deserve to be considered married. Either they have a license or they don't, and if they do, they get to make medical decisions, pick a child up from school or file a joint tax return without facing any questions about their sexual practices. Basic fairness dictates that same-sex couples should be able to formalize their relationships without having to pass a moral test that is only imposed on them.
Without a doubt, this law represents a change from the way things were when most of us were growing up, but that's not reason enough to maintain the status quo. There is a cultural shift afoot, which is why polls show that young voters are much more comfortable than their parents with changing the marriage laws.
Sure, this is new. But the values behind it - fairness, privacy, tolerance of people who are different than you - are not new and have long traditions in Maine.