Probably because they worship satan, the father of lies, and whatever satan does is in direct defiance of God (think about our upside down world, where evil is called good, men are women, etc) and being trans or having a trans "wife" is going against God's intended design for creation.

Think about it - humans are in direct defiance of God due to sin. By going along with God's intended design (man with woman), you are thereby submitting yourself and your will to God. However, those who have aligned themselves with satan will be doing the will of their father, who the Bible says is the father of lies: "You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies." (John 8:44)