My advice, which isn't worth very much but hey, is to be yourself.
It may take a long time but hopefully they'll come around once they get over the impact of the news and their own prejudices.
Worst case is that they ardently refuse to ever consider your happiness or needs and would choose to "expel" you from the family unit. Well frankly, if that's their attitude it's probably not a relationship worth saving anyway. Mind you, I've always been independent from my family, so I have no idea what it's like to be tightly bonded into a family unit.
Sorry for your angst...I hope it will work out!
J.