Some masculine traits in women are accepted to some extent. But, look at the backlash against that Algerian boxer.

For someone who really cares about fitting in with society, the pressure to conform can be pretty brutal. There's probably more freedom to be who you want to be now than ever before. In the past not only gender roles, but every role in society was extremely rigid. People didn't even have the freedom to decide whether or not to wear a hat outside. The expectation was that everyone wore a hat, and if you didn't you were a real oddball.

I strongly suspect that some of the people who think they're trans are just people who have interests/passions/attitudes/personalities that don't conform to their stereotypical gender roles.