It is because we fight against what God declared in Eden. Remember,Christ rebuked the Pharisees saying "but from the beginning, it was not so." When we reject one of the Father's commandments from Eden, the others eventually follow along. We rejected the Father's decree that the husband is the head of the home first. We rejected the idea that it was God that united man and woman in marriage, and thus what God put together, man should not put asunder through divorce. Now we are rejecting the idea that God created them male and female.
We cannot reject the words of God the Father in part. Once we open that can of worms, we must either fully repent, or in time consume it all the way to the dregs.