Very good news to hear, although I'm sure those who live in the eastern part of the state will do their best to fight it. While the remaining liberal/progressive states will probably make gay marriage legal(or at the very least civil unions), I feel it will be a long time before the same is true on a national level. Luckily the acceptance for gays/gay marriage is growing, but there is still a long way to go. My family is pretty socially conservative, and they can't realize how I can be straight and pro-gay marriage(my parents are at least okay with civil unions though).
Tell them about Loving v Virginia (or is it the other way around?), the ruling that made interracial marriage legal in all 50 states and how many conservatives used the same arguments then that they're doing now.
I have, they claim that that ban was unconstitutional as it limited the amount of people of the opposite sex one could marry. They claim that marriage has always been between one man and one woman, and that all the instances I bring up of it not being true don't count. My family, and those on my mom's side, are Catholic while my dad's side are United Church of Christ(yet they hold conservative views), so I try to limit bringing examples from the Bible into the argument. They don't consider interfaith(which they are also against), intrafaith, and interracial marriages being acceptable, or polygamous and child marriages no longer being acceptable as the definition changing.
I've also told them (my parents)that if gay marriage lessens the value of their marriage that they need to see a marriage therapist.
In addition to not wanting to "change the definition of marriage", some also are against it because they think one chooses to be gay, or else those ex-gay groups wouldn't exist.
I wish I thought of this at the time, but a comeback to that would be: "If Christianity were the true religion, then people wouldn't leave it."