Why are transgender prayer requests "too far"?

It's going too far to pray for a sex change,

"God gives it a body as He has designed, and to each kind of seed He gives its own body.", 1 Cor 15:38

back up some verses and get the context: our physical bodies are like seeds...meant to become something else than what is planted.

God has a purpose for the body you were born with, e.g. a man born blind at birth was for this reason per Jesus Christ,

"this happened so that the works of God might be displayed in him.", Jn 9:3

Also,

"Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own", 1 Cor 6:19

The body's purpose is to house the Holy Spirit. Then, have the Holy Spirit transform your nature into God's image, or nature.

"until Christ is formed in you", Gal 4:19

Physical bodies are rather limited, but

"It is sown in dishonor, it is raised in glory. It is sown in weakness, it is raised in power.", 1 Cor 15:43

Our weaknesses are meant to drive us to God. None of our limits are His limits with us, though.

"out of weakness were made strong", Heb 11:34

Transforming our nature set us free from our fallen condition:

"you may be partakers of the divine nature, having escaped the corruption that is in the world through lust.", 2 Pt 1:4

'Lust' is desire for anything we aren't qualified or meant to have (not just sexual in nature). Wanting to be something other than what God made you to be thwarts His plans with you:

1. be transformed into His nature, thus set free from corruption
2. have the most fulfillment in life, and
3. have a personal relationship with God Himself

Also,

"He created them male and female", Gen 5:2

Non-binary or any other gender identification is contrary to God's Word, thus 1, 2 and 3 above would never happen.

NOTE: corrective surgery is OK. That makes the body given to you more functional. But, simply trying to become something other than what God made you is not yielding to the Holy Spirit's purpose. In fact, it is rebellion,

"But who are you, O man, to talk back to God? Shall what is formed say to Him who formed it, “Why did You make me like this?”", Rom 9:20

"“Woe to those who quarrel with their Maker, those who are nothing but potsherds among the potsherds on the ground. Does the clay say to the potter, ‘What are you making?’ Does your work say, ‘The potter has no hands’?", Isa 45:9

First, from a quote in your comment

"Tell me that it is rational to talk to the Force of Being as though this were your closest confidant"

No, wrong. God is not a 'force'. Yes, Jesus instructs us to pray to God indeed as a Father. Matthew 6:9 is

"this is how you should pray: ‘Our Father ..."

Back to your question.

"every petitionary prayer is based on the premise that you know better than God"

No, wrong. God already knows what you need. Matthew 6:8

"your Father knows what you need before you ask Him"

Every petitionary prayer ought to be based on the premise that God already knows what you need. You are granted things when you are based in Christ. See James 4:3,

"And when you do ask, you do not receive, because you ask with wrong motives, that you may squander it on your pleasures."

Otherwise, God would not be sending a blessing. So, now to the specific question,

"Why is praying to change your gender "too far"?"

Presumably, the theologians you are referring to believe 'gender reassignment surgery' and so on is sinful. As a scriptural example, if cross-dressing is sinful, wouldn't 'gender reassignment' surgery and exogenous hormones also be? So for these theologians, you are praying basically for God to be complicit in sin.

So, according to them, it is a wrong-headed attempt to get something that would make your life worse off. Instead, seek first the Kingdom and its righteousness, and all you need will be added to you, as in Matthew 6:33.

"But seek first the kingdom of God and His righteousness, and all these things will be added unto you."

Question

You cite 2 examples of prayers that (according to you) are based on the premise that we know better than God:

1. Because you're poor, you judged that God didn't give you enough money in your life, so you ask for more
2. Because you're an unbeliever, you judged that God made a poor decision in His divine election, so you pray that you will be converted

So you ask why, on the same principle, shouldn't a transgender also pray to change their sex since God made a mistake?

Answer

In the first two examples, the premise "we know better than God" is wrong because

• the corollary that God made a poor decision or made a mistake is not necessary
• things are not set in stone, and God wants to be asked

Here are typical right bases for the first two requests:

1. We are poor. But we also know that

   • Jesus wants us to pray for "our daily bread" (Matt 6:11)
   • Jesus teaches us to rely on God in faith for our needs, and to not worry that God neglects us, because God certainly cares for us more than the birds and the lilies in the field (Matt 6:25-34)

   so we pray in response.

2. We are currently an unbeliever. But the right understanding of divine election is that

   • God wants everyone to be saved (1 Tim 2:4)
   • God takes into account our free will, and election is based on his foreknowledge of our free will
   • The elect become believers at different points in their life, and some experience fully the unbelieving stage and the struggle they face to become believer

   so not knowing the future and not knowing whether we are the elect we pray, because the praying itself is part of the experience of the elect.

Why the transgender prayer is "too far"?

The first two examples show we ask within the range of what God allows, but most Christians believe that sex reassignment surgery (SRS) is not allowed in most cases. They may recommend (if applicable) that the transgender pray for their gender dysphoria to be healed. This is STILL a currently controversial issue that should be approached with carefulness, compassion, and faithful Biblical interpretation. A recent book that does all 3 is Affirming God's Image: Addressing the Transgender Question with Science and Scripture (2019) by a Professor of Christian Ethics J. Alan Branch (Amazon link here, review here, critical review that dispute some of his Biblical interpretation here).