If we're being honest, I'd be disappointed. I'd love to have grand kids I guess the traditional way. I'm no where near being a father stage though.
However, if it were to happen. You take it on the chin. It was your choice to have kids, not theirs. You do the right thing, and love them no matter what. It wasn't there choice to come into this world, and they had about as much choice about their sexuality as they did the colour of their eyes.
Disowning a child because of it is cowardly, and is a sign of low testosterone in men.