However,being an adult human male, or "Man" is a biological fact. It is not an opinion, or a whim, or a desire, or a choice, or a social construct.
It's kind of both.
Male and female are descriptions given to babies at birth based on their genital appearances.
It's also a description of gender, i.e. the roles that people act in society. The gender role does not need to follow the biological role.
Although in your opinion, it obviously must. I assume you don't apply such rigid role models to other sphere's of society. For instance, I don't suppose you have any problem with a meritocracy, or a soldier espousing a pacifist role, or a priest taking up arms, etc.
It's just the sex angle that gets you triggered.
Now when you say, "a woman becoming a man, I assume you are talking about a permanent change of gender.
Because transitioning is not a temporary process, it's a permanent process. Although mistakes can be made.
If you are referring to the temporary process of a woman temporarily acting out the role of a man, that would be theatre, or cross-dressing, etc.
But transitioning isn't just a whim, or a temporary desire, it's a life-changing decision. And that case it is acceptable to call it a choice, or an opinion.
It's a choice or an opinion of how they wish to live the rest of their lives. And legal processes exist to allow that.
So being a man or a woman can be both a biological label and a social construct. And the social construct can be interchangeable.
The biological definition can, to all intents and purposes, also be interchangeable.
As of yet, it's not been possible for a biological man to have the whole biological functions of a woman. But there are many women who can't have babies. It doesn't make them any less of a woman.