Recent news has once again brought to the forefront the issue of "transgenderism".  This phenomenon is based on something called "gender theory", and it is a critical issue that far transcends arguments about who can use which bathroom. Society is being pressured to accept something about the very nature of the human person that is fundamentally false and dangerous
The whole idea of "gender theory" is, in my opinion, so nonsensical that it is hard to believe that anyone actually accepts it. The argument is that "gender" is not determined by one's biological sex, but is a separate matter that is defined according to the subjective desires of an individual. To these advocates, one's biological sex is an arbitrary classification that is "assigned" at birth, and has no intrinsic connection with one's actual sexual identity
This is a bizarre and dehumanizing idea.  It denies the unity of body, mind and soul, and it rejects the logical and scientifically undeniable understanding that biological sexual difference is essential to human nature.  It treats the body as a mere physical shell that can be used or manipulated as one wishes. People become just raw material, to be fashioned and changed — and mutilated, as happens with "gender reassignment surgery".
This ideology is contrary to reason and science, much less faith.  Science has proven beyond any reasonable doubt that sexual difference is a significant part of human biochemistry, physical structure (not just our reproductive system, our brains too), behavior, and psychology.  It is also at the heart of an authentic Christian anthropology, which recognizes the inherent complementarity of the sexes, and their dignity as man and woman, made in the image and likeness of God
Pope Francis directly confronted the danger of gender theory in his encyclical on respect for reation, Laudato Si.  In his typical pithy way, he said:

Learning to accept our body, to care for it and to respect its fullest meaning, is an essential element of any genuine human ecology. Also, valuing one's own body in its femininity or masculinity is necessary if I am going to be able to recognize myself in an encounter with someone who is different. In this way we can joyfully accept the specific gifts of another man or woman, the work of God the Creator, and find mutual enrichment. It is not a healthy attitude which would seek "to cancel out sexual difference because it no longer knows how to confront it".

Several years ago, in his annual address to the Curia, Pope Benedict went even deeper and confronted the philosophical flaws in gender theory, and its larger significance (my emphasis is added in bold):

[T]he question of the family is not just about a particular social construct, but about man himself—about what he is and what it takes to be authentically human

The Chief Rabbi of France, Gilles Bernheim, has shown in a very detailed and profoundly moving study that the attack we are currently experiencing on the true structure of the family, made up of father, mother, and child, goes much deeper. While up to now we regarded a false understanding of the nature of human freedom as one cause of the crisis of the family, it is now becoming clear that the very notion of being—of what being human really means—is being called into question. He quotes the famous saying of Simone de Beauvoir: "one is not born a woman, one becomes so" (on ne naît pas femme, on le devient). These words lay the foundation for what is put forward today under the term "gender" as a new philosophy of sexuality. According to this philosophy, sex is no longer a given element of nature, that man has to accept and personally make sense of: it is a social role that we choose for ourselves, while in the past it was chosen for us by society.

The profound falsehood of this theory and of the anthropological revolution contained within it is obvious. People dispute the idea that they have a nature, given by their bodily identity, that serves as a defining element of the human being. They deny their nature and decide that it is not something previously given to them, but that they make it for themselves. According to the biblical creation account, being created by God as male and female pertains to the essence of the human creature. This duality is an essential aspect of what being human is all about, as ordained by God. This very duality as something previously given is what is now disputed. The words of the creation account: "male and female he created them" (Gen 1:27) no longer apply. No, what applies now is this: it was not God who created them male and female—hitherto society did this, now we decide for ourselves.

Man and woman as created realities, as the nature of the human being, no longer exist. Man calls his nature into question. From now on he is merely spirit and will. The manipulation of nature, which we deplore today where our environment is concerned, now becomes man's fundamental choice where he himself is concerned. From now on there is only the abstract human being, who chooses for himself what his nature is to be. Man and woman in their created state as complementary versions of what it means to be human are disputed…. When the freedom to be creative becomes the freedom to create oneself, then necessarily the Maker himself is denied and ultimately man too is stripped of his dignity as a creature of God, as the image of God at the core of his being. 

The debate over "gender" is, at its heart, not just about bathroom access and kindness to those who have difficulty accepting their sexual identity.  It is about the nature of the human person and our relationship with the created world and ourselves.  The stakes are very high. We are fifty years into a massive social experiment that has revolutionized the understanding of sexuality based on a distorted view of human nature.  We are living with all its disastrous consequences. Gender theory takes us a bridge too far, to a place where human life loses its meaning.  And those of us who dissent are already facing legal pressures to conform.
A healthy society cannot continue to substitute falsehoods for truth, and expect people to thrive.. We must stand firm on the truth about man, woman and human nature.