What makes the US military so overwhelmingly dominant by international standards? 3 things: Preponderance (sheer size of the force), Technology (most advanced tools of warfare), and force employment (the capability and effective employment of our individual service men and women). How will admission of women into combat arms affect force quality? By direction of the Secretary of Defense every position that is currently closed to women will be reviewed and opened unless an exception to policy is requested and approved based upon the 5 Guiding Principles established by the joint chiefs of staff.
By these guidelines the arguments against inclusion of women based upon degraded social dynamics within units (fraternization, sexual tensions, expanded considerations in billeting or field operations) will have difficulty overcoming the SECDEF's standards for accepting gender based exclusion. Arguments against inclusion based upon the idea that force quality is necessarily degraded, however may hold if (1) lower standards for physical capability are applied to combat arms postions in order to facilitate the integration of women and (2) the lower standards necessarily diminish force quality within that particular field due to the nature of the position.
If lower standards are not applied to positions within the combat arms and the existing male only standards for physical ability are observed, then gender exclusion simply won't have a leg to stand on. If lower standards are applied to facilitate gender diversity in the combat arms positions currently excluded to women, then those fields that can best demonstrate a diminished level of force quality due to a fundamental dependency upon the physical ability of the individuals within that field will be best postioned to make an argument for gender based exclusion.
I would hypothesize that the spectrum for the strongest case for gender exclusion would begin with specialized forces ( Rangers, SEALS, RECON, PARARESCUE, etc.) and Infantry. This spectrum would likely end with the most mechanized combats arms fields (Tanks (the first five minutes is enough to get the idea), artillery) having the weakest case for gender exclusion...That is not to say that these fields cannot make a case at all but rather that physical capability is further from the core group of compentencies that determine force quality in the latter fields relative to the former.