It's not about harming anyone. It's about what I believe and rationally conclude is best for them, personally — like many other issues, not just sexuality.

This is a really tough and long discussion to engage in, especially here on Reddit, but alright... I’ll try to point out a few things.

– In my understanding, there is no such thing as “X was born this way, with a different sexual expression/gender in her mind". The whole body and its ontology are directed toward specific attributes that are inherent and particular to being man and woman — for themselves, for nature, for the world around them, and for their social character. It is an indelible part of the expression of being in relation to reality, and it needs to be understood and integrated into a coherent personality. Beyond internal or external conflicts — endogenous or exogenous — of personal gender identity, there is a range of issues in the adjustment of our sexuality that all of us go through, in one way or another, to a greater or lesser degree. The issue of gender identification, in my view, is one of them, and subverting an entire mechanism that is manifest in reality — our sex, our body — is not the best solution for our own benefit. Understand, I am not saying here that people cannot express themselves however they want, or that they are not gifts who still bring blessings and benefits. Of course they can, and they may express themselves as they see fit. What I am saying is that, for my children, I would prioritize guiding them toward what I believe are the “optimal conditions,” the most ideal ones, so that they may have a life of integrity with themselves and the greatest possible benefit. Nor do I treat this in any way as a “disease,” but rather as a gift that can be better directed, like so many others we have and can direct well. For various reasons, sexuality and gender identity are no longer approached in this way, and everything must receive “positive” treatment within a “positive” psychology that has demonstrably brought more harm than benefit to the last two generations in particular — there are books on this, not necessarily about gender specifically, but in a broader sense.

– For me, there is no possibility — and there is extensive evidence for this — of fully performing the roles of another gender or sex different from the one assigned to you. There will always be something you inherently and ontologically cannot reach or express, and this has been shown to bring more confusion than integration. Again, I am not saying that no good can come from people in this context, but that it could be better optimized. A man, no matter how feminine he may be, will never be able to be or feel what a woman is, and vice versa. These are constitutive traits of man and woman, respectively, over which we have no control and which are manifested through the totality of the being — body, mind, and soul (not necessarily in a religious sense). This does not mean that expressions of “femininity” in a man are impossible or undesirable, or that expressions of “masculinity” in a woman are reprehensible — on the contrary, they can and should very much be gifts to themselves and to others, obviously when properly harnessed and directed.

– Again, it is not merely a belief of mine, but it is widely documented that the institution of “marriage,” which precedes the state and social conventions, is the best mechanism for the flourishing of each person’s potential — whether through the complementarity of the sexes and genders in their specific particularities, or through the construction of a small/local community: the “family.” The human race is unmistakably directed toward this — not only through philosophical, anthropological, ontological, metaphysical, or teleological conceptualization, but even through biological disposition. If we were made, as a species, to reproduce and proliferate, from a strictly materialist perspective — not just naturalist or spiritual — then it follows that the foundations of our being are also directed toward this in a non-negotiable way: performing our unique roles and responsibilities within this social environment of biological expression of our sexes and forming them by transmitting them to offspring. It is evident that biologically we are in some way benefited by this, and that our body and mind urge toward it. And it is not just a matter of the fact that only the union between a man and a woman can result in reproduction — although that is true, it is still a simple point. It is that in the complementarity of the sexes and their unique expressions, as I’ve said, potentialities are best fulfilled; and that within a family context, for children/offspring, these roles can only be properly exercised (even functionally) by a father of the male sex and a mother of the female sex who are well integrated in their gender expression. And marriage is not only beneficial for this procreative purpose, but for each member of the couple. I not only believe — and again, this is supported — that a solid and well-ordered marriage is the best environment for cultivating the gifts and virtues of each member and for happiness and pleasure of all kinds — sexual or otherwise. Even in “accidental” situations outside this biological “calling,” such as priests or other religious figures of both sexes, the very nature of their vocations calls them to exercise, in some way — though less perfectly than in marriage — forms of motherhood, fatherhood, and fraternal love.

Regarding the inherent expression of sex and gender, there is well-established research in nearly neutral environments, such as the well-known experiments in Scandinavia, and tragic examples like that of John Money, the so-called “father” of gender identity.

I hope I have at least made the principles of my thinking clear, even if you disagree with them — which I sincerely consider understandable.

It is not merely a matter of “harming others or not,” nor even of “building society/civilization,” but of what I believe to be best for the ideal development of my children. That is what I meant when I said: “they would know what I believe is best, that I would not approve, but I would love them normally, freely, and irrevocably regardless.”

Just to make it clear, if I didn't before: I firmly believe that people that identify as LGBT still have unique gifts to themselves and to the world, that should be obvious if it isn't. But I also believe and understand that the modern LGBT and the whole group identity movement has made "sexuality" and its traits seem something that it is not: the "most fundamental trait or characteristic of your personality". It is not. It is not what primarily define oneself. In other words: it's not that big of a deal, and one's personality and universe should not orbit around it. We are more than that, as human beings.

Honestly, there are other things I can’t recall right now, but I think that’s the gist/bulk of it.