Bame women, suffering with the double whammy that they do, are perfectly entitled to decide what feels personally worse when it comes to discrimination, sex or race. (If a distinction is even possible, which I should imagine is often quite difficult).
The point is, the law regards them both equally, in terms of protected characteristics.
This woman was discriminated against on the basis of her sex. She was considered so lesser than, that a man wouldn't, couldn't even sit next to her.
No one, certainly not the airline, should even be contemplating a solution to this man's prejudice - other than sit down or get off.
But on a practical level I still don't understand the question others have asked: why is the woman asked to move? Why isn't it the men who get moved? Serious, if dumb, question. I don't get it.
Sexism. Patriarchy, innit.