I don't think anything regarding sexual orientation should be discussed with children that young by their teachers. If parents want to discuss those things with their children, that's fine. Personally, I don't think a book with 2 dads is appropriate either, because it isn't natural. Children are conceived by a man and a woman, not 2 men and not 2 women. If a child is being raised by 2 dads or 2 moms, then it's up to them to explain to their children why that is. When children get older, it's perfectly fine for such subjects to be discussed if they raise questions about things of that nature in class.
Public schools are for teaching our children the vital things they are going to need to be a productive member of society, such as English, math, science and history. Sexuality, values, political beliefs and religion are things that kids learn, discuss and/or adopt from their parents, and public schools (pre-high school) need to stay out of it.
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