Transgenderism is the belief that someone can be the gender opposite their sex. The justification proffered for this definition is that sex is a social construct, and so therefore it can and should be reconstructed to make people happy. Constructs of male and female have been reconstructed so that people are categorized based on preference, not biology. Cultural issues arise from this notional shift and the justification for it. I will be outlining some of these issues here.
Accepting the logical justification for transgenderism leads you to unacceptable conclusions
Most Liberals agree that you can identify as the gender opposite your sex because gender is a social construct, and social constructs should be changed to make people happy. Following this logic necessarily leads you to the conclusion that the woman in this MyTake Norwegian woman claim to be born as a cat? is in fact a kitty cat. Why? Cats, like gender, are socially constructed. Our criteria for what constitutes a cat has been established by society, hence the term "social construct". If you redefine our sexual constructs to make others happy you must be willing to do the same for cat-ladies, or man-children (also mentioned in this users MyTake). Most Liberals do not find this conclusion acceptable, and yet that is where their logic takes them.
Accepting transgenderism tarnishes the utility of sex-related pronouns
Words are symbols we give meaning to for our communicative convenience. When I use a word it should be clear based on its definitions and the context in which I am using it, what meaning I am trying to convey. Words lose their utility when they have the potential to convey x meaning and their antonym simultaneously. The confusion this creates leads to less efficient social interactions, as more layers of investigation and/or elaboration will be required to discern the intended meaning of the type of words described.
For our purposes, think of the words "male" and "female". If we lived in a Liberal utopia, I could tell my friend that I had sex with a girl I met at the bar last night, and to truly discern what I am saying, he would have to ask me: "a girl with a real or synthetic vagina?" We make words for our communicative convenience. We should try to avoid the communicative pitfalls that come with redefining sex on an identification basis.
Accepting transgenderism leads to gender-abolitionism
Gender-abolitionism is what it sounds like: the stance that gender pronouns should be erased completely. This stance is justified by the logic that although men and women are different at the aggregate level, on an individual basis a female can fit the social, male prototype and vice versa. In other words, you can have manly women and feminine men. This is also going to be true on a biological basis, when new technologies enable a biological man to become pregnant. At this point sexual descriptors supposedly become useless, because people with x y chromosomes will have no pertinent differences socially or biologically from those who have x x chromosomes. It follows that gender-laden words should be abolished on the basis that these words no longer describe a reality idiosyncratic to people of different sexual-chromosomal compositions.
The issue with this argument - which I have fairly represented - is that it is of course not true. Even in this technologically advanced world there will still be differences between men and women based on their DNA. One important difference being that of a real vagina/penis vs those which are synthetic. I don't know about you, but I wouldn't want to have sex with someone who has a synthetic vagina, just as I'm sure most women wouldn't want to have sex with someone who has a synthetic penis. There would be many other important, residual distinctions to be made along the lines of these chromosomal differences, but that is a topic for another paper.
What is important to know is that the acceptance of transgenderism leads society down this undesirable path. If you give credence to the notion that sex is something which should be premised on what people identify as, you detract from the importance of the biological and social distinctions that gender-laden words possess. This obviously gives credence to the gender abolitionist argument, which is already seeping into our mainstream institutions. Just ask CNN and their "people with cervix's". https://www.cnn.com/2020/07/30/health/new-cervical-cancer-screening-recommendations-wellness/index.html
Transgender people have a mental illness that isn't getting the attention it deserves.
Mental illness doesn't have an established definition, but one criteria that is agreed upon by people who put forth plausible definitions for the term "mental illness", is that of self-harm and harm to others. People who claim transgenderism isn't a mental illness state that simply being transgendered is not harmful; it only presents a harm when the trans-person is experiencing depression alongside their gender-identity. Without these negative emotions, simply being transgendered is not a mental ailment they claim. Of course, this is completely inaccurate.
Transgenderism is debilitating in our society. The concepts of male and female describe a biological reality that trans-people cannot attain. When they masquerade as the sex opposite their gender they confuse themselves and others. Go back to the bar example. Let's say you bring a trans-female home from the bar and she presents to you her elbow fat, penis. Is this debilitating to the self and others? Yes. I don't think I need to describe how this scenario plays out, and it is only one of many ways in which just simply BEING transgender can be debilitating, even without the association of negative affect coming into the picture.
Because society refuses to acknowledge that being transgendered is a mental illness, we are not doing the research necessary to help them with their condition. They have a misconstrued interpretation of their surroundings, like someone who has schizophrenia. This misconception causes them harm in our society; and yet, we ostracize and disparage anyone who even acknowledges that we should be treating transgender people for delusions, discouraging them from pursuing treatment research that could cure them of their problem.