Do you have any insight into why, when they were looking at men vs. two types of women, 88% of the blood donors were men? Seems like you would want at least 50% women.
Using a small sample exposed to the suspected risk and the vast majority not exposed sounds like responsible ethics.
It is not stated, but there should also have been additional safeguards and monitoring. It is normal for studies to be terminated early if they are found to expose participants to either too much danger or if the study group is found to be significantly advantaged (in which case, for example, the whole group is given the beneficial treatment instead of just the small study group).
50% may sound 'fair' or 'equal', but this is about researching ways of saving life, some social notion of fairness or gender equality is not relevant and, if this study is correct, could have caused excessive death. Is it worth people being killed to get the gender balance into the shape you prefer?
This is obviously a tough question to answer, and maybe the answer here is no. Further, I wouldn't say "worth people being killed" as much as I'd say it's "worth risking people dying". Obviously this is context sensitive and is different for each treatment and study.
Despite this, as the devils advocate, you could say that given that men and women can respond to different treatments in a range of ways, the ideal distribution would be diverse enough to accommodate for those differences, which would provide a better statistical subset and by extension producing "better" data. Same caveats apply, where you mentioned "too much danger" and if the study is "significantly advantaged". In short, it could be less about fairness (equity) or equality and more about collecting a broader or more diverse range of data.