I think a couple distinctions need to be made. There’s a difference between banning something and not providing it. Tolerance of evil can be ok if a greater harm could come from forcibly suppressing it. With that out of the way:
Public authority exists to protect and advance the common good. That’s the role of government (cf CCC 1898, 1927).
So what is the common good?
CCC said:
1924 The common good comprises “the sum total of social conditions which allow people, either as groups or as individuals, to reach their fulfillment more fully and more easily” (GS 26 1).
1925 The common good consists of three essential elements: respect for and promotion of the fundamental rights of the person; prosperity, or the development of the spiritual and temporal goods of society; the peace and security of the group and of its members.
Note in that passage, the common good is not just about material prosperity, but also spiritual. Man’s salvation is a consideration. As St. John XXIII wrote:
Pope St. John XXIII:
- In this connection, We would draw the attention of Our own sons to the fact that the common good is something which affects the needs of the whole man, body and soul. That, then, is the sort of good which rulers of States must take suitable measure to ensure. They must respect the hierarchy of values, and aim at achieving the spiritual as well as the material prosperity of their subjects.(42)
- These principles are clearly contained in that passage in Our encyclical Mater et Magistra where We emphasized that the common good "must take account of all those social conditions which favor the full development of human personality.(43)
- Consisting, as he does, of body and immortal soul, man cannot in this mortal life satisfy his needs or attain perfect happiness. Thus, the measures that are taken to implement the common good must not jeopardize his eternal salvation; indeed, they must even help him to obtain it.(44)
Furthermore, since all authority comes from God, government cannot use means contrary to God’s law and must take His law into account when determining what measures to take:
CCC said:
2244 Only the divinely revealed religion has clearly recognized man’s origin and destiny in God, the Creator and Redeemer. The Church invites political authorities to measure their judgments and decisions against this inspired truth about God and man:
Societies not recognizing this vision or rejecting it in the name of their independence from God are brought to seek their criteria and goal in themselves or to borrow them from some ideology. Since they do not admit that one can defend an objective criterion of good and evil, they arrogate to themselves an explicit or implicit totalitarian power over man and his destiny, as history shows.51
So in sum, the role of government is to defend and advance the true common good using morally licit means.
As I mentioned before, this does not mean outlawing every sin. Tolerating contraception use could be justified in some circumstances and outlawing it could be justified in other circumstances. However, ordering its use or ordering that it be provided cannot be justified.
Given the definition of the common good, I don’t see how abortion could ever be tolerated since it always infringes on “the fundamental rights of the person” and by definition does not allow those aborted to “reach their fulfillment more fully and more easily”