Well, yes, there's a gender spectrum: it's fine to be a girly dude, or a buff lady, or whatever, but there is a smaller segment of people who are (in simplified terms) born with a brain that's gendered differently than their body.
For those people, becoming and fully expressing who they are is essential, even life-saving. It doesn't hurt anyone else: it's just about them. No petty personal or cultural biases of ours should stop them.
The transperson I'm closest to comes from a very healthy, loving family, has a wonderful marriage, is a sensible, reliable caring leader at her job. It's time to stop calling transpeople less-than or mentally ill and just accept them as humans who are different than us.