The world is hell and everyone is going to loving die. If you can't handle somebody near and dear to you thinking your love life is strange without considering cutting them out of your life entirely, you need to figure out what your priorities are.
my priorities: enjoying my life and not dealing with people who want me to enjoy it differently
"thinking it's strange" is not what we're talking about here and you're intentionally trivializing it. OP's grandfather is thinking about taking a
plane trip to see them so that he can presumably give them a firm talking-to about their gender identity. that's not thinking it's strange, that's an obviously-unwelcome attempt to push your way into someone else's personal affairs, which have nothing to do with you, because you think it's wrong somehow
I've dealt with this very problem many times in my life, living in a conservative state with a very religious family. Sometimes you have to thicken up and bite the bullet, or let them be completely ignorant. Preemptively cutting somebody out of your life on a whim with no closure for either sides makes YOU the starfish.
if you are making my life worse, I'm not gonna keep you in it, period. you're gonna know why (I don't know where this "no closure" stuff came from, or "preemptively" for that matter, I literally said "if someone's giving you a hard time") and then maybe one day you'll get to thinking and realize you've been a richard your whole life and let me know that you've changed but until then I just don't care. I don't owe you anything just because you're my uncle or the father of my mother