One of the biggest reasons I hear from parents why they don't discuss gender and sexuality with their children is, "they're too young to understand." What I hear with that response is, "it's not something I understand or am comfortable speaking with them about." Children will learn anything we teach them.
There is a false perception of what life is like today for lesbian, gay, bisexual, transgender and queer high schoolers in America. Yes, it is certainly better than it was for me 35 years ago in Nebraska, but every high school isn't a great, or even safe, place for every student. Not every kid is alright.