Not exactly. The idea that law can be arbitrary is a severe and tyrannical perversion of the law. Human law is a circumstantial determination of the moral law (or ethical principles, if you will) and the demands of justice. Lex iniusta non est lex (an unjust law is not a law). So we are not at complete liberty to legislate arbitrarily just because we think we have the power to do so. Indeed, we run the risk of passing laws that aren't laws at all, but only have the appearance of law.

Furthermore, determining the law is not the same as determining the outcome of having that law on the books. The law is permissive about many immoral behaviors, at least in part because the enforcement of laws regulating such behaviors would cause greater harm than the behavior in question. Prudential judgement among lawmakers is necessary to determine what should and should not be regulated.