Two things.
One: The suffix -phobic means "afraid of", for example arachnophobic means "afraid of spiders". So the literal meaning of "homophobic" is "afraid of homosexual people".
Two: I first learned about LGBTQ in year 4 or maybe year 5, and this is how I learned about it. This is also my entire experience with LGBTQ so far.
One of my friends would always have a "crush" on some guy, and I was like "sure whatever" because it did not make any difference to anyone's life.
Then one day, she decided to have a crush on 7 or maybe 8 people, some guys and some girls. I was like "Excuse me what the heck". She explained LGBTQ to me and said she was [insert sexuality here, depending on the day].
She/he/they/it then called me "homophobic" because I thought having a ridiculous primary-school crush on multiple people at once was stupid.
She also called me an "uncultured swine" for not knowing what some sexuality meant.