I think it depends on the significance of the action. Some actions we regard as immoral but not terribly important. The person sitting next to me in the library on the computer is annoying me giggling at Facebook with her companion, and I suppose this is immoral, since she has a duty not to disturb others in a public space in this way, but hardly something that should be illegal, although the idea of her being hauled away by the police is not an unpleasant one to me.

For liberals it is the notion of harm that is significant. If you think that abortion causes harm then it would be difficult to argue that the law should not be involved. Of course, marital infidelity also causes harm and there are countries where it is harshly punishable. Another highly relevant issue is whether one thinks that punishing behaviour will be effective in controlling it. So one may think an action is immoral and should be punished by the state, but it is not likely to be effective to try to do so. Which means there is no point in making it illegal.