I can't say that I would just brush it off and not think anything about it, it would probably take a little time to get used to it, but I would support my children whatever they decided to do. My OH has a lot of gay people in his family (his sister and 2 cousins) and I also have a gay cousin and my grandma has admitted to being bisexual.
OH's dad doesn't speak to his daughter since she told him, although I think there was more to it than just finding out she was gay (I think the fact that she got drunk and announced it to all his staff on their xmas do) but I would never let it effect my relationship with my children, I will be there to support them.