With great trepidation I offer to act as an intermediary of information that many here will find to be offensive, bigoted, and vile.  Many may find it personally injurious.  But I hope that this will be received as evidence that some people out there have a completely different worldview than you do, and that they have not flippantly decided to oppose transgenderism.

First, allow me to quote from a [major Evangelical Christian statement on sexuality](https://cbmw.org/nashville-statement):

> WE DENY that adopting a [...] transgender self-conception is consistent with God's holy purposes in creation and redemption. (article 7)

> WE AFFIRM that it is sinful to approve of [...] transgenderism and that such approval constitutes an essential departure from Christian faithfulness and witness. (article 10)

Adoption of this particular statement on sexuality was debated by the leaders of at least two large Protestant denominations in the US in the last year.  Tens of thousands of person-hours were devoted to debate and discussion on whether these statements and others in the statement are true.

For another example, consider [this document](http://www.vatican.va/roman_curia/congregations/ccatheduc/documents/rc_con_ccatheduc_doc_20190202_maschio-e-femmina_en.pdf) published by the largest Christian "denomination" in the world, the Catholic Church.  It is not as quotable as the previous, but argues that transgenderism is not in alignment with Natural Law and thus two thousand years of Church teaching:

> Gender theory (especially in its most radical forms) speaks of  a gradual process of  denaturalisation, that is a move away from nature and towards an absolute option for the decision of  the feelings of  the human subject. In this understanding of  things, the view of  both sexuality identity and the family become subject to the same ‘liquidity’ and ‘fluidity’  that  characterize  other  aspects  of   post-modern  culture,  often founded  on  nothing  more  than  a  confused  concept  of   freedom  in  the  realm  of   feelings  and  wants,  or  momentary  desires  provoked  by  emotional  impulses  and  the  will  of   the  individual,  as  opposed  to  anything  based on the truths of  existence. (19)

> This ideology inspires educational programmes and legislative trends that promote ideas of  personal identity and affective intimacy that make a radical break with the actual biological difference between male and female. Human  identity  is  consigned  to  the  individual’s  choice,  which  can  also  change in time. (22)

> The process  of   identifying  sexual  identity  is  made  more  difficult  by  the fictitious constract known as “gender neuter” or “third gender”, which has  the  effect  of   obscuring  the  fact  that  a  person’s  sex  is  a  structural  determinant of  male or female identity. Efforts to go beyond the constitutive  male-female  sexual  difference,  such  as  the  ideas  of   “intersex”  or  “transgender”,  lead  to  a  masculinity  or  feminity  that  is  ambiguous,  even though (in a self-contradictory way), these concepts themselves actually presuppose the very sexual difference that they propose to negate or supersede. This oscillation between male and female becomes, at the end of  the day, only a ‘provocative’ display against so-called ‘traditional frameworks’, and one which, in fact, ignores the suffering of  those who have  to  live  situations  of   sexual  indeterminacy.  Similar  theories  aim  to  annihilate the concept of  ‘nature’, (that is, everything we have been given as a pre-existing foundation of  our being and action in the world), while at the same time implicitly reaffirming its existence. (25)

Perhaps you read that and you don't see the connection to this debate – after all, this is just over the use of pronouns and being polite to others, not individual opinions on whether transgenderism is legitimate or not.  But recall that most Christian traditions uphold the Ten Commandments, and many traditions interpret the ninth commandment as explicitly forbidding the purposeful expression of something that is false.

Using stated pronouns in opposition to biological sex then becomes a matter of truth or lie, and the CoC, from their perspective, requires them to lie in certain circumstances, which is a direct violation of what God commands them to do.  And his commands matter more to them than SE's do.

Now, the main thing you gather from this might be that a lot of Christians are bigots.  But hopefully you will also get a glimpse that many people who hold positions that are clearly anti-transgender have done so carefully.  How much "good faith" they deserve is, of course, a completely different question.

***Note** that I have no interest in debating the merits of the views contained in this post in the comments.  I have attempted to accurately relay information in response to the question asked.  Feel free to advise me if I have inaccurately portrayed the viewpoints of these groups, and I will strive to correct the error.  If you have further questions about the viewpoints of various Christian traditions on transgenderism, I encourage you to visit Christianity.SE.*