It’s the same probability of throwing 5 heads in a heads and tails coin toss. GCSE maths.
If men and women were rewarded with a nomination equally then the probability of the first nomination being a man would be 0.5 or, in percentage terms 50%.
The chances of it being 2 men out of 2 nominations is 0.5 x 0.5 (or 25%). The percentage chance of all five nominations being men is (0.5 x 0.5 x 0.5 x 0.5 x 0.5) 3.125%.
So there’s a roughly 97% chance that this situation would not happen.
Therefore it is biased. Is it biased because men are so much better at music or that men are more likely to be nominated? Which do you think? Either way, for women to get a fair chance of being nominated, they need their own category. Unless you think women shouldn't deserve a fair chance.