Pacem in Terris II - State Authority is Limited

This isn't a surprise, and it's hardly the only example of this notion in Catholic teaching. Pope John XXIII tells us that governmental authority is not unlimited, and consequently rulers are not free to make just any sort of laws that they wish.

Governmental authority, therefore, is a postulate of the moral order and derives from God. Consequently, laws and decrees passed in contravention of the moral order, and hence of the divine will, can have no binding force in conscience, since "it is right to obey God rather than men."

Indeed, the passing of such laws undermines the very nature of authority and results in shameful abuse. As St. Thomas teaches, "In regard to the second proposition, we maintain that human law has the rationale of law in so far as it is in accordance with right reason, and as such it obviously derives from eternal law. A law which is at variance with reason is to that extent unjust and has no longer the rationale of law. It is rather an act of violence." [PT §51; quoting Acts 5:29 & ST I-II, Q93, A3, ad 2]

A critical phrase here is "right reason". That doesn't necessarily mean that if I consider a law to be irrational that I can just ignore it! The assumption there would be that my reason is equivalent to right reason, and that might be an invalid identification. Given that it is difficult for men to reason rightly in the first place, it would be an arrogant presumption for me to blithely hold that a law is irrational (and therefore unjust) on the basis of what I think alone. We're much better off to not think too much of ourselves. So it's not that there are no situations in which laws are unjust, but rather that we dare not presume that we are competent to make such judgments on our own authority.