Military Forces Assuring that Treatment of Items by Gender are Uniform and Equal across the Services Act of 2021 or the Military FATIGUES Act of 2021 This bill requires the Office of the Under Secretary of Defense for Personnel and Readiness within the Department of Defense to establish consistent criteria for determining which uniform or clothing items across the military departments are considered uniquely military for purposes of calculating the standard cash clothing replacement allowance for members of the armed forces. The office must review the established criteria every five years and recommend adjustments to enlisted clothing allowances if they are insufficient to pay for uniquely military items. Additionally, the office must periodically review uniform clothing plans of the military departments to identify data and requirements necessary to facilitate cost discussions and recommend adjustments, items in the military departments' calculations of the enlisted standard cash clothing replacement allowances to develop a standard by which to identify significant cost differences that need addressing, plans of the military departments for changing uniform items to determine if the changes will result in significant out-of-pocket cost differences among the departments or among genders, and initial officer clothing allowances to identify requirements needed to facilitate cost discussions and adjustment recommendations.