Benefit or harm
Weighing of the benefit or harm of each alternative is the only rational basis upon which a decision may be made, because maximizing benefit to society is the ultimate goal of all morality. (In other words, there may be other good ways to make decisions, but they can't be called "rational".)
This is only true, however, if the sum total omits:
- the harm of the original act
- the sum of all present and future harm resulting from the act or rule
Obviously we can't know the sum of all present and future harm, but can only make the best possible guess by examining the evidence of decisions on similar questions. This process, and its results, is then the best possible proxy for actual foreknowledge.
If the moral values upon which that original act is judged are in turn based on reasonable arguments, those arguments may become part of the discussion; if they are based on dogma, doctrine, or other "revealed truth" (i.e. argument from authority), then the discussion can go no further since the premises are hidden inside a black box.