And if patriarchy didn't cling to the idea of normative binary opposite gender roles - man = male =masculine & woman = female = feminine - then there wouldn't be a problem. A born man who wanted to adopt the lifestyle, behaviour, looks, dress of a woman could just do so. Just as women today wear trousers, have short hair, earn money, own property, and vote, as men of yesterday were the only ones alowed to.
And if men are ridiculed - well, think of the ridicule (to the point of violence) directed at women fighting for liberation over the milennia.
But to say a man who lives as a woman "becomes" a woman - well, it's a misrepresentation of biology. But if they're doing what they want, living how they want to live, who cares?
It's mostly men who are bothered about men staying men but adopting a "feminine" lifestyle (for want of a better word). It's a mix of misogyny and homophobia.
Because homophobia is founded on misogyny. To stereotypical "masculine" men a man who loves men, and has sex with men, and identifies as gay, is akin to being a woman, and therefore less than fully human.
Wasn't it Professor Greer who said ages ago, that women have to overlook how much men hate them? Feminism is dangerous knowledge in that way.