In order to learn about sex you’re going to have to learn about either straight sex, gay sex, or both. Turns out both types of sex happen to exist. If you’re going to learn about straight sex you should probably learn about gay sex too in case you’re gay. If you’re trans the topic of puberty and your changing body gets 10 times more complicated. You should probably learn about that in case you’re trans. It would be helpful for queer kids and the things they experience to not be left out of sex ed, also so that kids who don’t know there’s some form of queer yet don’t grow up thinking that they’re broken.
Since both straight and gay sex exists, and because trans people exist too, learning about sex from those perspectives is important.
This kind of education is what could have stopped an entire country from thinking that AIDS is caused by being gay.
Excluding queer sexuality and gender from sex education is what we’ve been doing since forever, and surprise it’s left queer kids unprepared for the things they will encounter, and the other kids with the kinds of assumptions that you express here.