From a biological perspective, the women being banned here are not just men and as far as I'm aware cannot realistically compete in the men's division any more than any other woman. Practically these changes bar women athletes with certain medical differences from competing in the Olympics.

I'm not an expert so idk whether that's fair or not but that's what this decision is doing.

It would not be fair, because the point of having divisions is allow women to compete in a competition that is not dominated by men.

For most of the sports there is no men's division - it's open for everyone.