It's not uncommon for parents to pull out all stops to prevent or otherwise derail a transition. Whether through ignorance, fear, denial, anger, bigotry or a mix of all, some will be quite shameless in their actions.
It's one of the reasons why I didn't tell my folks until the very week before I went full time...I figured if I told them any earlier they would see it as an opportunity to talk me out of it. My mother, despite initial massive reservations and fears has been very supportive, my father is able to tolerate me but isn't accepting.
It's a lot for parents to take in. Sometimes you just have to give them time to work through it themselves. It would be awesome if they jumped on board from the start but judging by the many, many posts I've seen on this forum over the years that is sadly rarely the case. But many do come around eventually too.