Have boys said they want shorts?
Yes, my son does and my husband campaigned to be allowed to wear shorts to work in the summer.
And anyway, why not just give the full range of choices to both sexes. ? That would be fair and equal.
Because skirts are not a valid or practical option for boys so it is an illusion of choice. Therefore you are, in reality, giving girls 3 choices and boys 2. So that is neither fair nor equal. And treating people equally doesn't mean treating them the same.
And to reflect your question back to you - how many boys are asking to wear skirts?