Except gender is a much less mature topic than sex and sexual attraction, something that should be taught around 5th grade if not earlier, before children have confusing feelings and their bodies undergo confusing changes. The media atmosphere already leaves impressions about sexuality in the minds of children much earlier, so there isn’t much harm for parents or teachers to discuss the topic in a much healthier way.
Gender on the other hand is something impressed upon and experienced by even toddlers, so the topic of gender isn’t all that mature, it’s only treated as mature because of the social taboos regarding trans individuals.