Why is walking in the flesh a bad thing if it is sinless?
It isn't.
Walking in skin and bones is not the same thing as walking in the "flesh". that is defined as a worldly oriented mind
Here is what Paul teaches water baptized Christians-
Therefore do not let sin reign in your mortal body, that you should obey it in its lusts. Romans 6:12
Obey and live !
Just because the vessel is mortal does not make it sinful.
Jesus had a mortal body, and it wasn't sinful.
So did John the baptist, but he had the Holy Ghost from his birth.
The sin in the mortal body of a born again water baptized Christian, desires to rule over us; to make us its slave.
A slave of unrighteousness.
There is no sin in the born again vessel.
It has been gendered by the seed of God !
So unless you are accusing God of providing Himself a sin filled temple to live in, it is time to change your POV.
Paul teaches that we are to present the members of our body as instruments of righteousness.
Correct, and he is not commanding the impossible.
And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive from the dead, and your members as instruments of righteousness to God. Romans 6:13
Right, and so do I.
We will receive new sinless bodies when we are resurrected from the dead, which occurs at His coming.
We will receive new vessels when we are resurrected, but we already have bodies worthy of the Holy Spirit's residency now !
Can you same one single sin a body can commit ?
If not, why accuse it of containing sin ?
But now Christ is risen from the dead, and has become the firstfruits of those who have fallen asleep. For since by man came death, by Man also came the resurrection of the dead. For as in Adam all die, even so in Christ all shall be made alive. But each one in his own order: Christ the firstfruits, afterward those who are Christ’s at His coming.
Your error is you were taught falsely that the resurrection of the dead occurs at water baptism instead of at His coming.
Not at all.
Water baptism in the name of Jesus Christ for the remission of past sins entails our death, burial, and resurrection with Christ from Christ's grave.
"Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
5 For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:" (Rom 6:3-5)
I hope that clears things up for you.