hi nicole
i think you're asking the wrong people if you want to hear "don't tell them! live a lie your whole life!"
basically i regard your situation as a normal part of growing up. everybody realises at some stage that they have their own life to lead. nobody is the perfect child, and i think you can judge parents as to how well they modify their hopes for their children and gradually grow up as parents themselves - "okay, i wanted my son to become a top lawyer in senile, senile & senile, but i'd rather he were happy and healthy and will support him in it, even if he ends up working at macdonalds".
so what i want to say is, you seem quite well adjusted (what a strange term!) from the short post you wrote, so i imagine that your parents did something right bringing you up, and i imagine that they love you, and you love them. and seeing as they don't have problems with other gay relatives, chances are, things will turn out fine, if you tell them.
of course, they probably already know. loving, attentive parents aren't quite as slow as many people like to believe
and if they don't already know, try spending demonstratively more time with your girlfriend. they'll proabably soon catch on.
hope this helps.
howie