the existence of many acceptable cultural practices does not imply that any cultural practice would be acceptable.
judging many options to be acceptable and then reaching the conclusion that any option is acceptable is called the many / any fallacy. to illustrate this fallacy, consider documentation styles for computer to a program; that does not mean that any commenting stlye is good.
it is false that all possible cultural practices have equal legitimacy. certain practices must be forbidden and others must be mandated if a socciety is to survive. this observation leads us directly to our next point.