"Our task is to teach our children about human sexuality according to the law God has written in our hearts."

Yes it is and Jesus spelled this out:

"“Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’ So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”"

In Genesis we read:

"So God made man in his own image, made him in the image of God. Man and woman both, he created them. And God pronounced his blessing on them, Increase and multiply and fill the earth, and make it yours ... "

"That is why a man is destined to leave father and mother, and cling to his wife instead, so that the two become one flesh. Both went naked, Adam and his wife, and thought it no shame." 

Then came the Fall .... and sin entered creation.