Should women serve in front line combat units? Yes, and the question is anachronistic.
Women now serve in front line combat units and “front line” does not acknowledge the nature of the battle-space of our time. Sadly and problematically, today’s battle-space lacks the more clear definition of front lines of past conflicts. Flying close air support missions, flying missions where man-portable air-defense systems or MANPADs pose a threat of death or capture or engaging in operations and activities in environments where ambush and improvised explosive device detonations are the nature of war today place women squarely in the front lines; or, more accurately, in the front space. That will not likely change, and the women who have served competently, coolly and valiantly in that environment affirm their position in the battle-space of today.
If the question is intended to address more specifically serving in front line ground units, the answer remains yes. Like the women who have served brilliantly in our other military services and specialties, the key is adhering to the relevant, legitimate, and unwavering physical, mental, and intellectual standards of a particular military specialty. That is a must, and my experience is the women who seek that demanding path will not want it any other way. Similarly, women who strive and fail must be viewed in exactly the same light as male aspirants who do not succeed.