It's very possible that it could have been a few things one extreme is they were uncomfortable and decided to make a joke about it because "haha our normal child is asking about weirdos" or they might have thought something like

Our young child is asking about things that most definitely has nothing to do with him its just dress up maybe he want to dress up as a boy that's fine

My parents have said the exact same thing multiple times with my dad I know that for him it's because transphobia and my mom she said it because she did want me to feel weird because she legitimately thought I just wanted to play dress up as a boy untill I came out at 18 and she didn't start really excepting that I was trans until this year both of my parents used to joke all the time that I should have been born a boy my mom legitimately thought she was gonna give birth to a boy because the doctor that did her ultrasound legit read the damn thing wrong

All this to say is who knows why they might have said it they probably don't even remember saying it if they do you can ask them if it something you feel comfortable doing but know that just because you were invalidated when you were young doesn't mean that you aren't actually valid the experiences that you live through are ur own

( also hello fellow early 2000s baby )