If people do not let their children learn about things like this, how are the adopted children of gay couples ever going to be accepted at school when they explain that they don't have a mum, but have two dads.
I can kind of understand the parents view, but i think most children are more intellegent and understanding than parents think. If they parents say 'you can't read about things like the because it is wrong' then they child is more likely to grow up homophobic, a simple explanation should be enough to satisfy the childs curiosity, and in a society that is becoming increasingly tolerant of homosexuality, i think it is important that children are educated too.
all my opinion of course... feel free to disagree
