I'll tell you what my parents told me. No, I do not think they're good reasons, but it's what they believe.
I do not share these beliefs with my parents.
They believe that you are born with a "truth" about what type of body you have. They don't believe hormone replacement therapy actually changes what gender you are. Sure it can have impacts on your behavior and body type, but if you stop taking them you'll revert back to "who you really are" (their words) "like when an alcoholic stops drinking they return to their natural normal self."
Because being a woman and giving birth are so tightly related in their mind, if you can't give birth post-op then whatever you had done was self mutilation at the hands of your doctor. Arguments about AFAB women that aren't fertile don't punch holes in their logic, because it's not logic.
They simply do not have thoughts about women transitioning to be men. That's a tomboy. They really only despise men who transition to become women as an impossibility, lying to themselves, and lying to everyone else "and we're supposed to play along with this obvious lie?"
Ultimately they don't believe anything you do changes that "fact" that was determined about your body when you were born. My dad will smuggly say that he wants to transition to be a frog. He wants surgery to make him more frog like and pills that make him think like a frog, but will be a frog? No, he'll be an asshole.
There are holes in their logic left and right because they aren't using logic. They've made the decision using emotions and feelings and backfill support for those beliefs however they can. My parents don't do it to be controlling, that's not their intention at least. It's probably because they themselves have not struggled with their identity. My dad, a man, has never questioned being a man. Being a man feels natural to him. To hear about somebody else transitioning probably makes him momentarily think about him transitioning and he doesn't want to do that. That's normal, BUT he overestimated what is natural about this feeling and instead of realizing it's a personal decision he believes people who have or are transitioning are unnatural.
Oh shit, she failed to multiply by 10 accurately. I might have been able to forgive them if a 7 was involved, but 10?!?!