What happens to mans penis when becoming transgender?

Answer 1

If they merely become transgender, and are not getting surgery to stop being transgender or transsexual, then nothing happens to it. It is there as long as they wish to keep it. They may tuck it or wear special garments to hide it.

For a transsexual person, they are born that way. Some transsexual folks mistakenly think they are transgender. Or they turn to the transgender community, accept it's rhetoric, and accept medical care under the transgender label, even if they find that to be insulting, because they see that as the only way they will be able to transition. So the typical goal for a transsexual person is to stop having transsexualism and to get their body (physical sex, maleness or femaleness) corrected to match their inborn gender identity, and to fit in with the mainstream, cisgender community without having transsexualism anymore. It is much like getting cancer surgery, if your cancer goes into full remission, then you don't consider yourself a cancer patient any longer, let alone a part of some community of cancer patients.

So in case the asker really wants to know how surgery to eliminate transsexualism is done, then the penis is pretty much turned inside out and grafted inside them at the new location, with as many of the nerves and blood vessels preserved as possible. The "spongy" parts of the penis along with most of the erectile tissue is removed, and the urethra is shortened to a female length and sutured into the new location below what will be the clitoral hood. The testes are removed and the scrotum is used to form the labia of the vulva.