Sadly morality seems to have gone out the window. Many people simply view things as legal or illegal.
And the reason should be obvious. What is legal or illegal is well defined, and if you are in doubt, you can look it up in the proper places. On the other hand moral and immoral are subjective, depending on the ethical system you accept. The same act is immoral in one system and neutral in another one. Probably the best example is sexual behavior.
As to which of your numbered items, I go for natural law (which is not the laws of nature, as some seem to believe!).
Many people do. The best argument against this comes from observing children. They act instinctively, just like what their
human nature compels them to do. Only children can love you unconditionally, or torture other beings without any qualms. If you have not read Orson Scott Card’s novel: “Ender’s Game”, I strongly suggest it. Without giving away the plot, the basis is that only children can be as vicious as the worst sociopath - when they are playing and not aware of it.
Oddly enough, Catholic moral teaching is more founded on natural law than the other way around. This is because we believe that God is good, and so what He has taught us to do is good rather than decrees from on high, more like the owner’s manual of a car than the diktat/whims of an all-powerful ruler.
I understand that this is your opinion, but it is not convincing to others. God’s goodness is not obvious. Actually, if anyone would behave as God (allegedly) does, the last adjective that would come to our mind is “good”. Most likely it would be “indifferent or uncaring”, though “evil” might be a possible choice, too.
If you can look at the world with an unjaundiced eye, and consider reality, the picture will be rather grim. All the natural features, which do not have to be there, and many of which we can change - speak loudly about indifference. And, no, you cannot shrug it off and blame our “fallen nature”. When there is an all-knowing, all-powerful, all-loving deity, EVERY act is his responsibility - either directly or indirectly. And there is no difference between them. But this is another conversation.
Basically, if you do as your owner’s manual suggests, your car will serve you better for longer, right? And some of those maintenance items, like fixing your brakes, are required by law for the maintenance of public safety, right?
No doubt about it. But, of course, you can
test the suggestions in the case of the car, and no such epistemological method is available for morality. Now there is also the minor question of “suggestions” vs. “commands”. Of course many of the Catholic teachings are excellent, and worthy to follow - because they are rational.