I'm not much older than you (just turned 26) but I've been realising more and more that all these ages I thought people were "grown up" or "more mature" or had that special thing I still lacked... well they don't. I'm growing up and I'm still just as lost having to make decisions than when I was 19 and moved out on my own. It didn't get any easier, sometimes it feels harder because when I was 19 I was at least following some kind of path, and now I'm out all on my own making decisions that are unique to me, and I can't look at any handbooks.
I definitely see the parent thing, although I would think it would be a concern when you're a teenager and dependent on them. Otherwise I would think that with parents who reject who you are, you're probably better off living without them. I am biased, as I have cut ties with my parents a few years ago for specifically these reasons. And I wish I had done it earlier. As good as it seemed to have their back if I needed financial help, a place to stay or medical help (they're doctors), it was actually more of a prison. I wish I had had the strength to affirm myself and stand for myself earlier.
Sometimes, I equate hanging on to your parents with hanging on to a bad relationship. You know the person is bad for you, but that's all you've got! They're abusive, but otherwise you'd be all alone! And if you have kids, well sure that guy is bad for them, but do you really have the right to deprive them of a father?
Well it's similar. If the potential grandparents reject you, if they're not willing to stand by you and support you, then they would NOT be good grandparents for your children, and your kids will be happier without them in their lives. And you might think it sucks for them, but I think it sucks less than going there and seeing your parents disapprove of you in front of them, or hearing comments, or seeing you distressed because you have to lie to them. And if you have kids, you want to set a good example for them, an example of being strong and independent, and willing to be yourself. If they see you sacrifice who you are and cower in front of your parents, what are they going to learn?
My mother's mother died when I was an infant, and I didn't see my other grandmother much, then she died too as I was a teenager. So I don't have grandmothers. I don't miss it. It's not like I have ever known it to begin with. And I think that's what you need to remember, too, you might think of it from the point of view of someone who has grandparents, and trying to see life without them, and having trouble with that. But it's not the same as not having any to begin with. Not to mention, if out of the three of you one set of your parents disown you, you still have two left.
And grandparents aren't always a blood thing. My husband doesn't have any grandparents that are biological, but he "adopted" a grandmother and a grandfather in some figures in his life, and because they chose each other they have a great relationship. If you do have children, you'll probably have plenty of friends they can grow up around, and they'll feel loved. I think you need to wonder "do I want person X to be around my child" basing it on who person X is, and not who they're related to.
Also, it seems a bit silly to base a fear of coming out on people who don't exist yet and might not for years, if ever. If you come out now and your parents take it badly, you still have time to show them you're happy this way, and for them to change their minds before you even have kids. If you come out later, when you do have kids, maybe then there will be a breakup and the kid will lose grandparents they actually knew already and might miss.
But yes, I definitely understand the problems that can come with coming out, and I'm not saying you should do it right now. Just that often, sentences starting with "I can't, because..." are just rationalisations, and the only reason you don't do it is because you're scared, and it won't get any better until it's done.