>>42485118
I don't know how should could be, except that she does not have, and to our knowledge never has had, any children. It would be pretty dark to bring up in a kids show though, since we know that apart from Flurry, no alicorn was ever born (Maybe Celestia and Luna were, but they get to tell everypony whatever the fuck they want since they are demi-gods who've been around forever), so maybe she has had a kid(s) and we just don't know it. I mean, she has a nephew somehow. Maybe there was a child or children born many generations ago and now that bloodline is just her "nephew". Still you'd think if Celestia took a lover and had a child, she would make the thing an alicorn. A mother's love and all that.
Alternatively she is sterile, or her long life has numbed her to the mortals and she just fuck around and aborts or puts them up for adoption or whatever.
It's hard to say, because like everything else in the show, Celestia got all fucked up as seasons went on. She was like a revered goddess figure in the beginning, and by the end that had completely changed. I suppose that happens though. It's actually kind of a natural growth if you think about it. In the beginning, when the show was young and it was an endless magical wonderful land that dazzled the eyes stirred the soul, Celestia was like mommy to all the little ponies. She was JUST that. The mommy goddess of a young child. As the show grew older, we began to see her in a different light. She wasn't infallible or all-knowing anymore. More and more we began to see that mommy is just a mare. I guess it's a lesson for the growing child, but it makes for really shitty epic lore. You know what? Headcanon whatever you want. It don't matter. None of this matters.
