Coming Out Question: Grandchildren

So, I'm a gay man who has yet to come out to my parents. He only person I'm out to is my uncle, who promised to keep it a secret. Unfortunately, he decided to take it upon himself to, and I quote him, "Test the waters for [me]." He asked my parents the hypothetical, "What if one of your kids was gay?"My Dad answered like I knew he would, with kindness and understanding and acceptance. My Mom had a different response. She said that she would feel, "Betrayed," because then she would be, "Cheated out of grandchildren." Which is pretty ridiculous, considering I have two straight sisters, but I digress.So, how should I respond to the grandchildren question? She is apparently vehemently against adoption because it, "Isn't the same!" (I should also point out that both of her parents and grandparents died before me and my siblings were born.) What should I say to help alleviate the blow when I come out?