It is no longer enough to simply allow consenting adults the freedom to do as they please in their private lives, because sex has become a public activity, a significant part of how we think about and communicate individual identity.
It is still considered good for a man to suppress his authentic self, if that self includes a desire for rape and murder. And there is little if any concern as to the damage one might to do himself by suppressing those desires. But if a man feels that he is not at home in his body, and is truly a woman, our society tells him that it would be harmful and even immoral in some sense to continue to live inauthentically as a man.
While you may have heard of Rousseau, Nietzsche, Darwin, and Freud, very few people have ever read their works, and yet their ideas resonate powerfully with millions across the globe, and their influence is seen almost everywhere from Disney to Playboy to your local school district.
With no perfect and unassailable view of the identities of the sons of God, daughters of men, and nephilim available to us, we must make do with the least-bad one.
Two of the most popular views of the sons of God from Genesis 6 are held with very little in the way of direct contextual evidence.