I posted this before, but having no military experience, I take this from my BIL, who led men into combat many times as a lieutenant, captain and Major across 5 tours in Iraq.
He said this: the biggest issue with women in combat is that men think differently when women are around. A male soldier might be briefly inclined to impress or defend a woman instead of focusing on the objective. Hesitation or that instinct can cost limbs or lives quickly.
He also said it was common that the women would essentially get "passed around" within the unit. This led to rifts between soldiers sometimes, not to mention the emotional toll it takes on the woman. The emotions, feelings, and dynamics that were unavoidable when women were in the unit were a liability to the safety of the soldiers.
That was his take, and it makes total sense to me.